This is the error message you get when someone else has the file open. Anychance someone else has opened the file and left it open on their machine?
Opening a FileMaker File directly from a shared drive is not recommended. It can result in a damaged file.
No, its not open anywhere else and I can open it fine on any other machine without Lion or FM Pro 11v4
Upgrades can reveal problems that are ignored in earlier versions. Or it could be a bug, but one would expect more complaints, if a bug.
Copy database to your computer. Run Recover under File Menu. Copy recovered database (differently named) to server. Test.
Having clean incremental backups of your database is a good idea.