Since the first upgrade from version 8.1 of Plesk, linux servers are having a strange anoying path issue related problems, like:

1. The first error message:
PHP Fatal error: Can't load Horde/SessionObjects.php, open_basedir restriction. in /usr/share/psa-horde/lib/Horde/IMAP/Tree.php on line 311

All you got to do to resolve this issue is:

Go to line 311 on your tree.php on the located path:
/usr/share/psa-horde/lib/Horde/IMAP/Tree.php

and change the following line:

require_once 'Horde/SessionObjects.php';
to
require_once './Horde/SessionObjects.php';

As a matter of fact, there are two lines to be changed, the other one is located a few lines down the 311.

2. The other error message received when you trying to modify/organized filters is:

Fatal error: Can't load Horde/SessionObjects.php, open_basedir restriction. in /usr/share/psa-horde/ingo/lib/Storage.php on line 276

The problem is solved the same way, just go to the line 276 of storage.php, and add "./" on the horde path as above.