Unexpected error when trying to add new theme

The reason for the error should be visible in the error log. You can find it in your hosting area. If necessary, the support of your hosting can help you.

Alternatively, you could also enable debugging in WordPress. How to do this is described here: https://wordpress.org/support/article/debugging-in-wordpress/ – also here the support of your hosting can help you if necessary to edit the file accordingly.

In both cases you should be able to see what the cause is in the logfile.

Yes, the hosting area is meant. And please open a separate topic for your issue: https://wordpress.org/support/forum/how-to-and-troubleshooting/#new-topic-0