Rolling back to an older version won’t resolve that permission issue.
It’ll be more helpful to verify what folder has been selected (if the target library has more than one) and verify the functional user running the Plex Media Server on your NAS has read & write access to that folder (user name depends on what version of DSM your running on that NAS → DSM 5/6 vs. DSM 7).
In each folder where PMS will be writing, there is a .grab folder, user PlexMediaServer must have Read-Write access to it.
It then most have read-write access to where you ultimately place the recording.
You need to do all this because Plex is not “Privileged” on the Synology.
It only has access where you allow (requested by MANY users and now mandated by DSM) AND only has the access level you expressly grant it.