Schedule Refreshed Failed ("Invalid Credentials" but authentification are correct)

Hello all! 

We have some kind of strang behavior with schedules at our Report Server at the moment. 

Their are Schedules who cant refresh because the SQL-Source Credentials are invalid. If we check the connection string we dont get any error. 

We use an Service Account and sometimes with different Reports we get this message and cant understand why. 

We tried to replicate the error when we planned a schedule for the same time with the same account but it worked well. 

Can somebody explain this behaviour or knows any solution for it? 

Thank you so much for your help!