Here are some suggestions that could work for you:
a) Check your process list for all entries that are referring to Plesk "pmm", for example
# ps aux | grep pmm
You will find a restore process running there. Kill it. Actually, kill all processes that are migration manager processes unless another valid process is running that you know of (e.g. a backup is running).
b) Check /usr/local/psa/PMM/tasks/ for a corresponding tasks file. Remove it if there is one, but do NOT remove tasks.db file.
c) From the longtasks table of the Plesk database, remove the corresponding restore process. It is possible that there is no entry on type like "restore", "pmm" etc., in that case do nothing.