Very strange behaviour. Probably you should check:
1. The consistency of the code. Does the file /var/www/web4/htdocs/mibew/libs/classes/Mibew/Asset/AssetManager.php really exists?
2. Permissions for all files and directories in the installation. Does the aforementioned file accessible for the web server?
3. Your caching solution. Maybe you should somehow manually clean some cache, or restart the webserver...
And as of the exception "Query failed: Table 'web2.plugin' doesn't exist", it means that Mibew Messenger wasn't properly installed. It's lacking at least one required table in the database.
p.s. BTW, the actual version of Mibew Messenger is 3.1.2, while version 3.1.1 contains one very annoying bug.