PMS can not play synced music anymore

Hello,

Running
PMS 1.9.3.4290
Android App 6.5.3.2115

I noticed that “some” of my music was not playing anymore.
After a while, I noticed that this occurs with albums that were synced (offline), other albums were playing fine.
Testen on LAN and also WAN of mobile.

Is this a bug?

kind regards,
Bart