This page is the collection of typical situations when checkpoint or restore fails.
PID mismatch on restore
If you see one of these lines in failed restore logs
Pid $number do not match expected $another_number
Thread pid mismatch $number1/$number2
this means that while restoring a process tree CRIU has failed to recreate a process or a thread with the PID (TID) value it used to have on dump. This most likely is due to the PID/TID value in question being busy with some other task or thread.