Hello. I get this same error if I delete a database and try to create another one with the same filename immediately after the delete. It seems that the server's cache holds onto the name for an unknown amount of time. I just tried this experiment today to see if I could get the new database created:
1 - issue dbcache flush at the server console several times (usually 3 is a charm)
2 - issue sh nlcache reset at the server console
3 - database created successfully
Previous attempts of issuing just the dbcache flush were unsuccessful. The combination of dbcache flush and sh nlcache reset seems to have worked for me. Not sure if this will apply to the issue you have, but it's something.