No reauthentication and downloads on iOS client

I’m using Seafile on iOS and Seafile on Raspbian. I had no problem adding and using the server on iOS. But after entering a library and downloading a file the problems begin. Whenever I reenter the client I don’t have to authenticate and when I try to download a file it fails.
Only after deleting and readding the server I’m asked to enter the password and I’m able to download a file.
Any idea what is wrong?