It must definitely be to blame that certificate is to blame, because so far nothing else runs on this server... I have already deleted the certificate and had a new one created, but the same result.
No idea what they want from me, if I click on https://pfiffikus.online I will still redirect to https://pfiffikus.onine... Try clicking on the link and you will see it.
and on my interface of the provider I see nowhere anything that could cause this
For starters, it would really help if you'd correctly spell your own domain name.
Secondly, there's nothing wrong with the current redirect (except for the missing HTTP to HTTPS redirect, but something that's not there cannot be incorrect at the same time ). Perhaps you need to clear your browsers cache?
Thirdly, a redirect problem is NOT a certificate problem. Redirects are send by the webserver. The existance (or absence) of a TLS certificate on its own does NOT interfer with any redirects what so ever.