Changes have been made in Git (master):

commit ad59737f3889414ad2b675d7ad2cbde84238cb7a
Author: Jan Schneider <jan@horde.org>
Date:   Thu Aug 9 20:54:15 2012 +0200

     Fix super global access (Bug #11277).

     PHP is optimized so that super globals are only made available if they are
     detected by the parser. Thus they cannot be accessed reliably as variable
     variables or $GLOBALS members.

  .../lib/Horde/Controller/Request/Http.php          |   22 ++++----------
  .../lib/Horde/Controller/Request/Mock.php          |   30 
++++++++++++++++++++
  framework/Controller/package.xml                   |    4 +-
  3 files changed, 38 insertions(+), 18 deletions(-)

http://git.horde.org/horde-git/-/commit/ad59737f3889414ad2b675d7ad2cbde84238cb7a

Changes have been made in Git (master):

commit af10bded7d53b7150a765ef813ce72b9485bb0d7
Author: Jan Schneider <jan@horde.org>
Date:   Thu Jul 19 02:32:18 2012 +0200

     Fix notice (Bug #11277).

  nag/app/controllers/CompleteTask.php |    3 ++-
  1 files changed, 2 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/af10bded7d53b7150a765ef813ce72b9485bb0d7

Changes have been made in Git (master):

commit 376e68ce06f804e7af69ee88c73069f9528896c5
Author: Jan Schneider <jan@horde.org>
Date:   Tue Jul 17 13:54:43 2012 +0200

     Cast to float (Bug #11277).

  nag/lib/Driver/Kolab.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/376e68ce06f804e7af69ee88c73069f9528896c5

Changes have been made in Git (master):

commit b6ad86d97e8a06e24c7c3a07e94061edeb9ad5ed
Author: Jan Schneider <jan@horde.org>
Date:   Tue Jul 17 13:43:58 2012 +0200

     Avoid notice if there is no parent class (Bug #11277).

  .../lib/Horde/Core/Controller/SettingsFinder.php   |    5 ++---
  1 files changed, 2 insertions(+), 3 deletions(-)

http://git.horde.org/horde-git/-/commit/b6ad86d97e8a06e24c7c3a07e94061edeb9ad5ed

Changes have been made in Git (develop):

commit ad59737f3889414ad2b675d7ad2cbde84238cb7a
Author: Jan Schneider <jan@horde.org>
Date:   Thu Aug 9 20:54:15 2012 +0200

     Fix super global access (Bug #11277).

     PHP is optimized so that super globals are only made available if they are
     detected by the parser. Thus they cannot be accessed reliably as variable
     variables or $GLOBALS members.

  .../lib/Horde/Controller/Request/Http.php          |   22 ++++----------
  .../lib/Horde/Controller/Request/Mock.php          |   30 
++++++++++++++++++++
  framework/Controller/package.xml                   |    4 +-
  3 files changed, 38 insertions(+), 18 deletions(-)

http://git.horde.org/horde-git/-/commit/ad59737f3889414ad2b675d7ad2cbde84238cb7a

But this is what we're hitting.

No, we don't. This is not a variable variable that we use, we just 
access the superglobals as items of the $GLOBALS array, which is 
perfectly valid and has always been.

Attached is the patch to fix the issue.

I'm still surprised it didn't break earlier for other users :)

Now I can mark items as complete in nag again.

Alright, the PHP manual clearly states this:

"Superglobals cannot be used as variable variables inside functions or 
class methods."
(http://www.php.net/manual/en/language.variables.superglobals.php)

-> We construct a variable variable on the fly inside the code.

I'm going to provide a patch for this.

The PHP installation is pretty vanilla, no special lock downs.

This might also be a "performance optimization" of newer PHP versions:
https://bugs.php.net/bug.php?id=55088

I just wrote this small test script based upon the Horde code:
-----------------------------------------
<?php
function getVars($name)
{
     return $GLOBALS['_' . $name];
}

echo "<pre>\n";
print_r(getVars(REQUEST));
print_r($GLOBALS);
print_r($_REQUEST);              // check result with this line commented out
echo "</pre>\n";
-----------------------------------------

The $GLOBALS array will only contain _REQUEST if the 
print_r($_REQUEST) is in place.

You have locked down or otherwise disabled the request_order PHP setting.

Changes have been made in Git (develop):

commit af10bded7d53b7150a765ef813ce72b9485bb0d7
Author: Jan Schneider <jan@horde.org>
Date:   Thu Jul 19 02:32:18 2012 +0200

     Fix notice (Bug #11277).

  nag/app/controllers/CompleteTask.php |    3 ++-
  1 files changed, 2 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/af10bded7d53b7150a765ef813ce72b9485bb0d7

The 'pear' driver was just a hack of mine to get it working -somehow-.
I've now reverted that hack.

Updating f.e. the description of a task is now possible.

But I can't mark a task as done via the UI. Nothing happens and this 
error message is in the logs:

2012-07-18T17:10:26+02:00 NOTICE: HORDE [nag] PHP ERROR: Undefined 
index: _REQUEST [pid 4151 on line 45 of 
"/datastore/DEVEL/horde/framework/Controller/lib/Horde/Controller/Request/Http.php"]
2012-07-18T17:10:26+02:00 NOTICE: HORDE [nag] PHP ERROR: Undefined 
index: format [pid 4151 on line 40 of 
"/datastore/DEVEL/horde/nag/app/controllers/CompleteTask.php"]

Those don't keep the task from being saved, but are fixed now nonetheless.
Not sure what you mean by that. By default the Kolab and IMAP backends 
use the Horde_Imap_Client library, not PEAR, and AFAICS there is no 
way to change that in the configuration. This is for a good reason, 
because the PEAR driver is not well tested.

Changes have been made in Git (develop):

commit 376e68ce06f804e7af69ee88c73069f9528896c5
Author: Jan Schneider <jan@horde.org>
Date:   Tue Jul 17 13:54:43 2012 +0200

     Cast to float (Bug #11277).

  nag/lib/Driver/Kolab.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/376e68ce06f804e7af69ee88c73069f9528896c5

Changes have been made in Git (develop):

commit b6ad86d97e8a06e24c7c3a07e94061edeb9ad5ed
Author: Jan Schneider <jan@horde.org>
Date:   Tue Jul 17 13:43:58 2012 +0200

     Avoid notice if there is no parent class (Bug #11277).

  .../lib/Horde/Core/Controller/SettingsFinder.php   |    5 ++---
  1 files changed, 2 insertions(+), 3 deletions(-)

http://git.horde.org/horde-git/-/commit/b6ad86d97e8a06e24c7c3a07e94061edeb9ad5ed

Hi,

updating an existing task with a Kolab backend fails with this error message:

2012-07-14T19:19:37+02:00 NOTICE: HORDE [nag] PHP ERROR: Trying to get 
property of non-object [pid 20767 on line 33 of 
"/datastore/DEVEL/horde/framework/Core/lib/Horde/Core/Controller/SettingsFinder.php"]
2012-07-14T19:19:37+02:00 WARN: HORDE [nag] PHP ERROR: number_format() 
expects parameter 1 to be double, string given [pid 20767 on line 306 
of "/datastore/DEVEL/horde/nag/lib/Driver/Kolab.php"]

Kolab backend driver used is 'pear'.

Cheers,
Thomas