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
- 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.