Login often fails

I have another strange issue. I frequently lose the function to login from the client. I get the error message that username/password is invalid and login fails is spite of having the correct credentials. To fix this, I have to go to the server, delete the user, and then add the user again with the same name and the same password.
Then login is then possible until next time the same thing happens, which is quite frequent.
73s - Ulf -SM0NOR