It will if error_reporting in PHP is set to output errors to the page.
It will be reported to wherever *PHP* is configured to log to.

I have finally been able to reproduce the blank page error consistently.

I recently received one email with a 8+MB attachment.
Every time I try to open this email in horde, I get a blank page.

Looking at the logs, there is nothing in either
/var/log/apache2/error.log
or in
/tmp/horde.log

/tmp/horde.log contains only login success and logout lines.
This is totally useless!

From this behavior it is clear that there is a memory limit that 
stops the execution to a
blank page.
Shouldn't the error be catched and an error message returned to the 
user instead of a blank page?

In my conf.php file, I have:
$conf['debug_level'] = E_ALL;
$conf['log']['name'] = '/tmp/horde.log';
$conf['log']['type'] = 'file';
$conf['log']['enabled'] = true;

$conf['fetchmail']['size_limit'] = '10000000';
$conf['compose']['attach_size_limit'] = '0';

So my questions are:
1) How can I disable the NO error_reporting() in horde?
  Where is the switch that would put back horde in testing mode?
2) Why the error(s) is(are) not reported in the logs (horde or apache)!

Please follow the hints at 
http://wiki.horde.org/FAQ/Admin/Troubleshoot#wsod to generate and find 
a useful error message.

When a folder contains too many message (not sure what the threshold 
is), I get a blank page (no error message, nothing) when trying to 
view the content of the folder.
When #of messages <1500, by navigating to another folder and coming 
back to the desired folder normally solves the issue.
In this particular case, I hadn't clean my Trash folder in several 
days and 4400 emails had accumulated. I was not able to access my 
Trash folder from Horde but only from a shell.