Suddenly 500 and 404 Error in one Category

[chtrede]

For some odd reason (I have not touched the sites backend for some time) since today one category is not displaying. Its category blog layout results in a 404 error, any item under the category in a 500 Error. I thought it had something to do with SH404SEF plugin, but even disabling it results in the errors. And, funny enough, sometimes the the category and articles work again, and then not...
I also disabled search engine friendly URLs for testing, but no difference. Some screenshots attaced. The only category not working is "Nachrichten".

Sometimes the very same menu item is calling /nachrichten, sometimes /nachrichte-news and then /aktuelles/nachrichten. But why that?

I also checked the server error log; empty.

Any ideas? Thank you.

[attachment=0]K1024_screen4_404-error.JPG[/attachment][attachment=1]screen1_404-error.png[/attachment][attachment=2]screen3_404-error.png[/attachment][attachment=3]screen2_404-error.png[/attachment]

[toivo]

According to the non-SEF URL, the id of the problem category is 90. Does that category exist in the categories table? Is it published?

[chtrede]

According to the non-SEF URL, the id of the problem category is 90. Does that category exist in the categories table? Is it published?

Nope, there is no category 90 and has never been an no menu item or so is allocated to such a non-existing category. I believe the whole issue is SH404SEF, allocation wrong URLs. Could that be?