Once again -Looks like problem is related with clearing the "Download Manager" data. After that I managed to pass the pending problem but then error code 963 appeared during the installation. I applied your fix (fastboot flash data userdata.img) and after that it was all fixed.
However relocking the bootloader is not convenient at all; first, I couldn't apply your fixes while the bootloader was locked... it all returned back with fail message "command is not valid at locked state"; second, locking the bootloader makes the Play Store problem reappear again (code 963) unless you unlock and reflash userdata.img again.
I think it's better to stay with unlocked bootloader again for the moment. I hope OTA updates will survive.
Thanks for your help and for the rom!
1. these ROM's you can flash on bootloader with any state.
2. it was not fixes, but format data partition. you should force format every time you locking or unlocking. otherwise - problems.
3. "command is not valid at locked state" - you tried to unlock without enabling OEM Unlock in developers options?
4. OTA should survive. It's up to you if you would like to have original stuff with 03 error. Pretty ... strange.
Just try to read post about error 963. Maybe this will clarify you what you did wrong.