can you check your "Media Cache" preferences, to make sure that it's not deleted too soon ?
Also, does this happen if you close the project and reopen it right after, or is it happening if you work on other project in between?
Hi and thank´s for your answer!
You are right, if I open the projekt very soon after having closed it I don´t have to render it again.
So I will do as you suggest, check my "Media Cache preferences".
Ok, this looks like a media cache issue (like your disk is full so older cache files needs to be deleted, etc...)
Hope these settings will resolve your issue.