On a new Roundcube (0.7) installation I recently did, something strange has happened: The defined plugins were not loaded and the error log was full with errors like these:

PHP Error: Failed to load plugin file /var/www/roundcube/plugins/lang_sel/lang_sel.php in /var/www/roundcube-0.7/program/include/rcube_plugin_api.php on line 196 (GET /)
PHP Error: Failed to load plugin file /var/www/roundcube/plugins/authenticate_by_confixx_email/authenticate_by_confixx_email.php in /var/www/roundcube-0.7/program/include/rcube_plugin_api.php on line 196 (GET /)
PHP Error: Failed to load plugin file /var/www/roundcube/plugins/persistent_login/persistent_login.php in /var/www/roundcube-0.7/program/include/rcube_plugin_api.php on line 196 (GET /)

At the begin I thought it could be a problem with the path, but it was correct (roundcube is a symlink to roundcube-0.7). Even by manually changing the vhost this didn't change the problem.
Another fact is that I use the exact same installation with the same configuration and plugins on another host, but one which is running on PHP 5.2.x.

Even by googling for this problem didn't find any clues, most of the hints were due to wrong paths or include path definitions.

But then it hit me! Just out of the blue I thought, what the heck, I check the permissions again and look at that:

/var/www/roundcube/plugins # ll
total 128K
drwxr-xr-x 4  501   80 4.0K Dec 22 10:20 acl
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 additional_message_headers
drwxr-xr-x 4  501   80 4.0K Dec 22 10:20 archive
drwxr-xr-x 2 root root 4.0K Nov  4 12:45 authenticate_by_confixx_email
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 autologon
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 database_attachments
drwxr-xr-x 3  501   80 4.0K Dec 22 10:20 debug_logger
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 emoticons
drwxr-xr-x 6  501   80 4.0K Dec 22 10:20 enigma
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 example_addressbook
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 filesystem_attachments
drwxr-xr-x 5  501   80 4.0K Dec 22 10:20 help
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 http_authentication
drwxr-xr-x 4  501   80 4.0K Dec 22 10:20 jqueryui
drwxr-xr-x 2  501   80 4.0K Dec 29 11:41 lang_sel
drwxr-xr-x 6  501   80 4.0K Dec 22 10:50 managesieve
drwxr-xr-x 4  501   80 4.0K Dec 22 10:20 markasjunk
drwxr-xr-x 3  501   80 4.0K Dec 22 10:20 newmail_notifier
drwxr-xr-x 3  501   80 4.0K Dec 22 10:20 new_user_dialog
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 new_user_identity
drwxr-xr-x 4  501   80 4.0K Dec 22 11:42 password
drwxr-xr-x 4 root root 4.0K Dec 27 13:12 persistent_login
drwxr-xr-x 2 root root 4.0K Oct 16 13:21 qtip
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 redundant_attachments
drwxr-xr-x 4 root root 4.0K Dec 22 11:38 settings
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 show_additional_headers
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 squirrelmail_usercopy
drwxr-xr-x 3  501   80 4.0K Dec 22 10:20 subscriptions_option
drwxr-xr-x 3  501   80 4.0K Dec 22 10:20 userinfo
drwxr-xr-x 4  501   80 4.0K Dec 22 10:20 vcard_attachments
drwxr-xr-x 2  501   80 4.0K Dec 22 10:20 virtuser_file
drwxr-xr-x 2  501   80 4.0K Dec 14 14:01 virtuser_query

Now isn't that something. All the plugins which were mentioned in the error log are the ones owned by root (I added them manually as they are not part of the original Roundcube installation).
As soon as I corrected the ownership of these plugin folders (and their files) / set them to the same user as the others, the plugins were loaded fine and no errors appeared in the error logs anymore.

Like I already mentioned, this worked fine on PHP 5.2.x but on PHP 5.3.x it seems to cause troubles.