I get notifications saying that eyebo uploaded a new map and if I click the notification I get redirected to the main page. When looking up his maps, it looks like he uploaded a KO3 map.
Maybe there is a problem with a redirect to maps that were uploaded in a certain group that isn't owned by the author?

He deleted the associated map.

Yeah, it was a Nadeo map I accidentally uploaded to my account.
I promptly deleted it.

I haven't uploaded any maps on purpose since my KO3 map 5 months ago. Good grief! Time to release a new map!

Thank you for the clarification. Will there be a function that deletes notifications to other players if they are no longer valid, or is the notification feature just implemented via a listener class? Then it would probably to much work to fix it

It's very possible (and makes sense), but probably not worth the effort for the time being.