SyncClient Login not possible

I already installed NC15 on Ubuntu 18.04 with MariaDB.
On an other Ubuntu 18.04 I have a Reverse Proxy fir different Webserver.
The login over works via web.

If I want use the desktop app an I add the account, the app found NC but after Login and Allow i’m in a loop (Login -> Allow -> Login -> Allow -> …)

I think there is a problem with the reverse proxy.
I tried some settings like this:

But It didn’t work.
Have someone an idea?
Thank you!