Usually Duplicati will use port 8200, so maybe some other service is hijacking port 8216?
If you have ssh access to the QNAP box, you can try netstat -tnlp to see which processes are listening where.
Dear kenkendk,
thanks for the quick answer. I solved the problem with reboot of QNAP NAS.
After the reboot of the QNAP NAS Duplicati 2.0.5.0 works, but me backup jobs were lost.
When this happens, it is usually because the new version is using another directory to store the file Duplicati.sqlite. I think this is related to QNAP, as Duplicati has not been changed with regards to where the storage file is located.