Lots of Horde and Pear apps from the stable branch generate errors... 
What's important is whether those are serious enough to stop Horde 
from functioning properly.

Horde requires to use some alpha modules in order to have access to 
all the functionalities (Holidays).

Here are some Pear and Horde examples:
HORDE LIBS
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_pgp::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Form_Type_image::onSubmit() should be compatible with 
Horde_Form_Type::onSubmit() [pid 13389 on line 3 of 
"/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_link::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_matrix::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_enum::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_mlenum::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Form_Type_mlenum::onSubmit() should be compatible with 
Horde_Form_Type::onSubmit() [pid 13389 on line 3 of 
"/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_set::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_sorter::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Form_Type_selectfiles::init() should be compatible with 
Horde_Form_Type::init() [pid 13389 on line 3 of 
"/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_assign::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Form_Type_assign::getValues() should be compatible with 
Horde_Form_Type::getValues() [pid 13389 on line 3 of 
"/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Form_Type_dblookup::init() should be compatible with 
Horde_Form_Type::init() [pid 13389 on line 3 of 
"/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of Horde_Form_Type_figlet::init() 
should be compatible with Horde_Form_Type::init() [pid 13389 on line 3 
of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Form_Type_invalid::init() should be compatible with 
Horde_Form_Type::init() [pid 13389 on line 3 of 
"/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Form_Type_country::init() should be compatible with 
Horde_Form_Type::init() [pid 13389 on line 899 of 
"/usr/local/lib/php/Horde/Form/Type.php"]
HORDE [horde] PHP ERROR: Only variables should be assigned by 
reference [pid 13389 on line 321 of "/usr/local/lib/php/Horde/Form.php"]
HORDE [horde] PHP ERROR: Declaration of 
Horde_Block_Vatid::_ajaxUpdate() should be compatible with 
Horde_Core_Block::_ajaxUpdate(Horde_Variables $vars) [pid 13389 on 
line 76 of "/usr/local/lib/php/Horde/Autoloader.php"]
HORDE [horde] PHP ERROR: Non-static method 
Horde_Core_Script_Package_Datejs::translateFormat() should not be 
called statically, assuming $this from incompatible context [pid 13292 
on line 87 of "/var/www/html/webmail/lib/View/Topbar.php"]

NAG
HORDE [horde] PHP ERROR: Declaration of Nag_Application::menu() should 
be compatible with Horde_Registry_Application::menu($menu) [pid 13389 
on line 811 of "/usr/local/lib/php/Horde/Registry.php"]

PEAR
HORDE [horde] PHP ERROR: Non-static method 
PEAR_Installer_Role::initializeConfig() should not be called 
statically, assuming $this from incompatible context [pid 14585 on 
line 599 of "/usr/local/lib/php/PEAR/Config.php"]
HORDE [horde] PHP ERROR: Non-static method 
PEAR_Installer_Role::registerRoles() should not be called statically, 
assuming $this from incompatible context [pid 14585 on line 45 of 
"/usr/local/lib/php/PEAR/Installer/Role.php"]
HORDE [horde] PHP ERROR: Non-static method 
PEAR_Installer_Role::getBaseinstallRoles() should not be called 
statically, assuming $this from incompatible context [pid 14585 on 
line 270 of "/usr/local/lib/php/PEAR/Installer/Role.php"]
HORDE [horde] PHP ERROR: Non-static method 
PEAR_Installer_Role::getInstallableRoles() should not be called 
statically, assuming $this from incompatible context [pid 14585 on 
line 271 of "/usr/local/lib/php/PEAR/Installer/Role.php"]
HORDE [horde] PHP ERROR: Non-static method 
PEAR_Installer_Role::getPhpRoles() should not be called statically, 
assuming $this from incompatible context [pid 14585 on line 272 of 
"/usr/local/lib/php/PEAR/Installer/Role.php"]
HORDE [horde] PHP ERROR: Non-static method 
PEAR_Installer_Role::getValidRoles() should not be called statically, 
assuming $this from incompatible context [pid 14585 on line 273 of 
"/usr/local/lib/php/PEAR/Installer/Role.php"]
HORDE [kronolith] PHP ERROR: Non-static method 
PEAR_ErrorStack::singleton() should not be called statically, assuming 
$this from incompatible context [pid 13389 on line 983 of 
"/usr/local/lib/php/PEAR/ErrorStack.php"]

KRONOLITH
HORDE [kronolith] PHP ERROR: Non-static method 
Date_Holidays::getInstalledDrivers() should not be called statically 
[pid 13389 on line 855 of 
"/var/www/html/webmail/kronolith/lib/Kronolith.php"]

This statement could not be more wrong.  Proof: you are apparently 
having issues with Crypt_Blowfish raising errors.  *NOBODY* who has 
used the stable version has ever reported any errors.

Well, it's a RC2, so should be fairly stable.

Sorry.  Meant to say 1.0.1.

https://pear.php.net/package/Crypt_Blowfish/download/1.0.1

Why are you using the beta version?  The stable version is 1.0.2.

Thank you.
I will, but the project seems dead. It's been 2 years since the last commit.

Btw, I've checked line 142 and it's
  if (!PEAR::isError($this->_crypt)) {

http://svn.php.net/viewvc/pear/packages/Crypt_Blowfish/trunk/Crypt/Blowfish.php?revision=302706&view=markup

This is an error in Crypt_Blowfish which is maintained on PEAR, not by 
Horde.  Report the issue there.

(Although I would really check to see if you are using an up-to-date 
stable version of Crypt_Blowfish. On my system, line 142 of that file 
is a PHP comment and PEAR::isError doesn't appear in that entire file.)

Crypt_Blowfish is not a Horde package, it's from PEAR. You should 
report this to that pacakge's maintainer.

I get a lot of errors like this in my logs:
HORDE [imp] PHP ERROR: Non-static method PEAR::isError() should not be 
called statically, assuming $this from incompatible context [pid 10924 
on line 142 of "/usr/local/lib/php/Crypt/Blowfish.php"]

but I don't know if this is symply a warning that can be ignored or if 
it's more serious.

Here is the full backtrace:
1. IMP_Contents_View->__construct() /var/www/html/webmail/imp/view.php:52
  2. IMP_Factory_Contents->create() 
/var/www/html/webmail/imp/lib/Contents/View.php:29
  3. IMP_Contents->__construct() 
/var/www/html/webmail/imp/lib/Factory/Contents.php:50
  4. IMP_Contents->_fetchData() /var/www/html/webmail/imp/lib/Contents.php:121
  5. IMP_Imap->fetch() /var/www/html/webmail/imp/lib/Contents.php:1510
  6. IMP_Imap->__call() /var/www/html/webmail/imp/lib/Contents.php:1510
  7. call_user_func_array() /var/www/html/webmail/imp/lib/Imap.php:364
  8. Horde_Imap_Client_Base->fetch()
  9. Horde_Imap_Client_Base->login() 
/usr/local/lib/php/Horde/Imap/Client/Base.php:2373
10. Horde_Imap_Client_Socket->_login() 
/usr/local/lib/php/Horde/Imap/Client/Base.php:725
11. Horde_Imap_Client_Socket->_tryLogin() 
/usr/local/lib/php/Horde/Imap/Client/Socket.php:399
12. Horde_Imap_Client_Base->getParam() 
/usr/local/lib/php/Horde/Imap/Client/Socket.php:603
13. Horde_Secret->read() /usr/local/lib/php/Horde/Imap/Client/Base.php:455
14. Horde_Secret->_getCipherOb() /usr/local/lib/php/Horde/Secret.php:92
15. Crypt_Blowfish->Crypt_Blowfish() /usr/local/lib/php/Horde/Secret.php:122
16. Horde_ErrorHandler::errorHandler() 
/usr/local/lib/php/Crypt/Blowfish.php:142