Hi Andreas, welcome to the forums.
If the rule is being shown in the UI then that means that there’s some part of it still in the rule jasondb that can be accessed by the api. If you know the uuid of the rule, you might be able to use the api interface in the UI to delete the rule.
If you haven’t changed anything else recently that you’re worried about loosing, then you might be able to restore the jasondb from the backup folder in
As a last resort if those two don’t work, you may have to resort to manually removing the stub of the rule from the jasondb file. This can be dangerous if you are not careful so I’d definitely try the other two options first.