[imp] Cannot download large folder

[Otto Stolz]

Hello,

I had written:
> One of my customers cannot download a large folder
> (“du” command reports: “81M”), whilst he has successfully
> downloaded two smaller folders. The user reports that
> he sees, whenever he has hit the “Download Folder(s)” button,
> after the “All messages ... will be downloaded” popup,
> a blank page.

> Grepping various configuration files for the catchwords
> “limit”, “size”, and “stor”, I have found:
> - in horde/config/conf.php:
>    $conf['session']['cache_limiter'] = 'nocache';
> - in horde/imp/config/conf.php:
>    $conf['fetchmail']['size_limit'] = '4000000';
>    $conf['compose']['attach_size_limit'] = '51904512';
> - in /etc/php5/apache2/php.ini
>    memory_limit = 128M
>    post_max_size = 8M
>    upload_max_filesize = 50M
>    session.cache_limiter = nocache
> - nothing in /etc/apache2/*.conf

> Now the question is, where is a size limit I should mend?

Chuck Hagenbuch has answered:
> I would try upping the memory limit first, to at least twice the size  
> of the largest folder that you want to be downloadable.

Alas, that has not helped much. I have enlarged the limit, stepwise,
up to 256 MB -- more than 6 times the respective folder's size (81 MB).
The PHP error log has the following entry:
> PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted
> (tried to allocate 81942820 bytes)
> in /var/srv/www/htdocs/webmail/horde/imp/lib/Folder.php on line 600

I have just checked in the IMAP (Cyrus) server's file system that the
folder in question does not have any subfolders (hence, a fortiory, has
no softlinks on itself).

What now, brown cow?

Thanks for any hint,
   Otto Stolz