Access databases are desktop databases, while fine for very light web usage are totally no good for a large forum like your own.
Access also shares 20 database connections shared with all websites on the same server who share the same App Pool Process.
Either your forum has outgrown Access or someone else on the same server sharing the same application pool is using up the small resource pool that you share for Access database connections, causing this error.
You could talk to your web host to try and resolve this issue, but my recommendation would be to upsize the Access database to SQL Server.