Unable to access Play Store after CM12.1 upgrade
When you flashed from CM 11 to CM 12 or 12.1, did you wipe /data,/system,etc. via recovery?  You'd also need GApps for CM 12/12.1 too, as that would include newer Google Play Services that are compatible with 12/12.1.

Generally when you upgrade from a different OS version (CM 11 being Android 4.4, CM 12/12.1 being Android 5.x.x), it is recommended fully wiping everything before flashing to prevent odd issues like you are seeing here.