Kopano Files: Error 401 when sharing via "share link"

Hi folks,
I am using Kopano with Nextcloud. In Nextcloud sharing via “share link” works without any problems, but if I use the Kopanofilesplugin -> click the file I would like to share and click on “share --> share by link” I get an Error 401 “Sharing failed”.

The logfile of Nextcloud says:

POST /ocs/v1.php/apps/files_sharing/api/v1/shares HTTP/1.1" 401 4948 

Cannot figure out, why the filesplugin “thinks” that this action is unauthorized.

One thing I also would like to mention is, that I access Nextcloud via SSL / Port 4430 (yes 4430).

Any thoughts and ideas are welcome.

Nextcloud version : 13.02
Operating system and version : Debian Stretch
Apache or nginx version: Apache 2.4.25-3+deb9u4
PHP version: 7.0.27-0+deb9u1

Kopanofiles: 2.1
Kopano Core:

I’m also running Webapp- and KC8.5.9 with nextcloud 13.0.2 but with PHP 5.6 cause I’m on debian 8- jessie
and I can’t confirm this issue.
Share with link works fine - maybe you can try an update to NC 13.03?
Is everythin else working? so can you access the nextcloud with click on Files?