Did you just upgrade from a version of PMS earlier than 1.15.4.994 ?
If so, please refer to this announcement. It contains the instructions to adjust for the change in how PMS works on Synology
Also, Linux uses / and not \ as its pathname delimiter. Be careful of context.
If you just upgraded DSM, it’s a good idea to check the permissions. DSM is known to ‘forget’ from time to time.
