Client asks for password very often

affected Nextcloud Client Version: different
affacted User OS: different
Nextcloud Server Version: 14.0.4
Server Operating System: Ubuntu Server 16.04

Hello everybody,
our community have a problem with the file synchronization with the Nextcloud Client.
Some of our users have to tap in their password multiple times in a period of 2 hours and grant access.
This problem occurs not at each user and at different Client Versions.
For login we use the User SQL App

At my client installation i got the problem only sometimes
If i use the Owncloud Client the problem occurs to

We use the user_sql app to authentificate our users. With a “normal” nextcloud user there is no problem

Have anyone an idea how we can solve the problem?

All I know is that on Windows it uses the network password function of the OS. I need tree re-enter the password when clearing Windows network passwords. But if it asks several users regularly it seems to be something with the server.

Have no one an idea?
The problem is the server
Some users told me that they have the problem too with WebDAV in Windows