Hi Dennis,
I'm sorry for this strange problem, and I want to give you all the support to solve it.
First,
to uninstall Tag Meta plugin, follow these instructions:
MANUALLY REMOVE TAG META PLUGIN
In plugin\system delete tagmeta.php and tagmeta.xml files
In administrator\language\en-GB delete en-GB.plg_system_tagmeta.ini file
In administrator\language\it-IT delete it-IT.plg_system_tagmeta.ini file
Also, check the plugin list installed from Joomla backend. If "System - Tag Meta" is present, you need to delete this row from jos_plugins table.
At this point you could try to reinstall just the plugin. If you still have problem, I can do it for you (if you trust), just let me know.
Regards,
Luigi