[thomas.rehberg]

P.S.: just found in this forum that you can force a connection test by simply clearing the cache. Will do that first to provoke a log-entry that can help. 
Might come back with a debug log in case I can generate something useful by that.

Well... it turned out that something must have corrupted my Kodi installation. I first tried with another qpkg for QNAP, Kodi 18.5, and it worked out of the box. Then I did a re-install of Kodi 18.8, and it works out as well.
No errors at debug level gave any indication, nothing that helped avoiding this brute force resolution. Maybe something went odd with the chrooted filesystem.
Happy now, just need to live with no finding but a clean install having fixed the issue.
Closed.