Hi,
thanks for the login data. I could solve this issue now.
I think it was a problem with an incomplete upgrade process.
What I did:
- installed
akeeba backup- created a complete backup
- installed the Joomla
eXtplorer extension
- then corrected the missing
Uncategorised folder in the jdownloads files root folder
- started a
monitoring scan in jD control panel to check the data consistences
- started finally the jDownloads
update to v.3.9.2 (this installed all missing parts from the first installation try)
- A first frontend check looks fine

Unfortunately I don't know exactly what was wrong with the first upgrade process and why it was interrupted.