Welcome to the forum @it-ruedi59
We’ll see if anyone who sees this can speak to a similar backup. Meanwhile, some general notes.
This is especially in the 90% to 100% range of the progress bar. If before that, it’s probably normal.
You can look at About → Show log → Live → Verbose if you like. It should get dlist then dindex.
When it starts downloading dblock it’s looking for some blocks that it hasn’t yet found. That’s slow.
Another thing that can slow things is too many blocks. Default blocksize is good for roughly 100GB.
Please change default blocksize to at least 1MB #4629 has more. Can’t change on existing backup.