Hi @corvaccio,
Sorry to hear of the problem you are having.
I have tested it and found the Wiki plugin is working fine on my test site.
Are you getting this message when you are create wiki page or default WordPress page?
Try deleting and reinstalling latest version of Wiki plugin.
This can be due to plugin/theme conflict.
To find find out plugin/theme conflict you can just try on your development site using default WordPress theme like Twenty Thirteen and deactivating all other plugins as displayed in the following flowchart to know which theme/plugin is conflicting if any.
https://premium.wpmudev.org/wp-content/uploads/2013/12/Support-Process-Support-Process.png
Kind Regards,
WPMU DEV