A Video-file is downloaded with youtube-dl based on mpd-file. However ffmpeg post-processor fails to convert it to playable mp4. The file downloaded with youtube-dl is encrypted. Is there a possibility to decrypt the mp4-file with the information in the mpd-manifest? Or are there other solutions to download and play PlayReady-protected-content?
The short answer: no, there is really no way to decrypt the content and save it on your disk as plaintext. That, I'm afraid, is the whole point of DRM.
The longer answer: in principle it is possible to play back the still-protected-but-offline content, if you manage to acquire the DRM license and if that DRM license contains sufficient playback rights (i.e. it's not expired nor it has other restrictions). But then you are forced to play it back with the same "agent" (loosely, the same application) that acquired the license -- for example, Edge or Internet Explorer or Windows 10's Movies & TV app (since the stream seems to be encrypted with Microsoft PlayReady).
All in all, not for the faint of heart, but theoretically possible.