You didn't follow the upgrade instructions. 
http://www.horde.org/apps/ingo/docs/UPGRADING

This is a support question, not a bug report.

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Hello,

After upgrading to Horde 5.2.11 from 5.1.x Ingo stopped working at 
several points.
One of these points is, that I can't add new filter rules by clicking 
on an e-mail address in a received mail and create a filter of it.
It appears an error message that the driver "Ingo_Script_I" cannot be loaded.

The most likely reason seems to be that in 
/ingo/lib/Factory/Script.php seems to be some confusion of variable 
contents.
It seems to be expected, that $scripts needs to be an array:

---------------
    public function create($rule)
    {
           ......

         if ($rule != Ingo::RULE_ALL && isset($scripts[$rule])) {
             $script = $scripts[$rule];
             $skip = array_diff(
                 array(Ingo::RULE_FILTER, Ingo::RULE_BLACKLIST,
                       Ingo::RULE_WHITELIST, Ingo::RULE_VACATION,
                       Ingo::RULE_FORWARD, Ingo::RULE_SPAM),
                 array($rule)
             );
         } else {
             $script = $scripts[Ingo::RULE_ALL];
             $skip = array_keys($scripts);
         }
---------------

But in my case, $scripts has the value (string) "sieve".

Since (const) Ingo::RULE_ALL has the value (int) 1
$script gets assigned the value "i", which later gets converted to 
uppercase and is then used as part of a class name.

I don't know if this is something that happened during an error while 
upgrading - but this file has the same mtime timestamp as all other 
PHP files in my installation, so I assume it got touched during the 
upgrade.