"File not Found" when creating new episode

Hello,
Ißm new to this forum and hope not to make everything wrong… But I have a problem with two Podlove installations on one Webspace: Without any changes, Podlove seems not to be able to find the files for new episodes. After having published 53 episodes on techtalk24.net and 10 episodes on campingglueck.de, suddenly new files are “not found”. I hanve´t changed the format, the folder in which they are stored or anything. Funnily, the duration can still be detected. What am I doing wrong? Hope is greatly appreciated.
Thank you
Bjoern