Fatal error: Cannot redeclare class MultilingualSection

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Hi everyone!

After the installation of the add-on Internationalization (which I had to install manually since I had problems with the normal installation) I cannot create new pages anymore. I got the "Fatal error: Cannot redeclare class MultilingualSection".

The first thing I tried was to uninstall the add-on, but after I input the okay for the uninstall, there is no confirmation of the actual uninstall and the add-on stays there.

What can I do?

Thanks in advance for your help