I tried out Polylang and after disabling it, all my categories are gone from the WP Admin Categories and Tags windows. They are still in the WP_terms table in database as well as attached to posts – posts already tagged / categorized still have the tags / categories in frontend and the archive pages for them work as they are intended to.
I can’t add any new categories – they disappear as well.
Reinstalling polylang did not fix the problem
What could be the reason why the categories and tags don’t show up in WP Admin and how could I fix it?
I have no idea how to solve your issue. If your terms are in database and are correctly displayed on frontend, then it means that either WordPress is corrupted or a plugin/theme is badly interacting with your admin. Of course if Polylang is de-activated, then Polylang does nothing…
I would try de-activating all plugins and switch to a default theme. Then if the problem persists I would try re-installing WordPress files via FTP.