I find that happens quite often for me, there are various "fixes" you can do, from closing things you are not currently using on your PC, to restarting the server or even (not recommended if you have a lot of work on it) resetting/deleting your server.
Also, it usually works if you play on the exact version the server is made for.
Edit: you should also try /save-all
Right after you create the new multiverse, that way, even if the server crashes, you will (hopefully) still be able to access the multiverse
about 1 month ago