Nextcloud Client stops syncing after Errors

Hi folks,
the Desktop Client stopps syncing after errors:
01.11.2018 07:20:43,Christian Schule/PPT9.ppt, nextcloud,Operation abgebrochen
01.11.2018 07:20:43,Christian Schule/PPT8.ppt, nextcloud,Operation abgebrochen
01.11.2018 07:20:43,Christian Schule/PPT9.ppt, nextcloud,Verbindung beendet

The upper two mean operation canceled, the third connection canceled.
The files are perfectly all right. A download via webinterface is no problem and I can open the files without any difficulties.
I want to change from owncloud to nextcloud and now I cannot get things working. Frustrating…

Server: 14.0.3
Client: Windows 10, Version 2.3.3 (build 1)

Any help is really welcome.
Best regards

Christian