@Niniann you forgot a step in your description, that is why it didn't work for
@D_Draco
Make sure you have a backup of your damaged project, if you don't have and the files in the new project are your only ones, get them out of that folder.
check the maximum number of the map files you have to restore (map ID, is in the file name)
Go to the new project where those maps are supposed to go.
create new maps until the map numbers are up to and include the map number you found above.
Do not do anything else on those maps!!!
Copy the mapfiles you want to restore into the project, overwriting the now existing blank maps.
Finished, the maps should now be visible although you'll have to resort them and recheck all transfers.