Profile picture upload

Hi,
I have a wordpress instance on a shred hosting system, running on plesk.

Wordpress runs smooth, also media upload.

When I want upload a profile picture, I get the following error:
2018-02-11 18:50:20 Error 62.216.207.230 403 POST /wp-content/plugins/ultimate-member/core/lib/upload/um-image-upload.php HTTP/2.0 … Mozilla/5.0 (Windows NT 10.0; WOW64; rv:58.0) Gecko/20100101 Firefox/58.0 220 SSL/TLS-Zugriff für nginx
2018-02-11 18:50:20 Error 62.216.207.230 21373#0: *1066078 access forbidden by rule nginx-Fehler

The ultimateusers upload temp directory is existing, has full privileges and I can access it if I place some file into it.
I put some error_log lines into files.php and it shows up properly:
[11-Feb-2018 17:27:11 Europe/London] Base:https://…/wp-content/uploads/ultimatemember/temp/
[11-Feb-2018 17:27:37 Europe/London] Temp:…/wp-content/uploads/ultimatemember/temp/

I have found similar problems from other users:
https://talk.plesk.com/threads/plesk-12-5-30-nginx-gives-403-forbidden-on-images-after-upgrade-from-11-x.336265/

Any idea how I can troubleshoot this?
I have tried to put some error_log() code into um-image-upload.php but it didn’t show up anywhere.
Any possibility ot use an alternative upload?

Thx a lot