Sage8.6 - Theme not found on PHP8.1

All my themes created with Sage8.6 are not working with PHP 8.x (and WordPress 6.1)
If the theme is activated I receive a critical error or the themes are not found (500).
Do you have an idea how to get the themes running again?

I stopped “developing” wp-themes so do I have to dig into Sage9 or Sage10?

Have you checked your server error logs? That should tell you where the problems are.

I found out it was not theme / sage8.6 related.
It was WordPress related I added some extra functions to my sage main template (into every theme extra.php) that cause that critical errors.