Looks like that big number of concurrent users is too much for Access to handle. I'd say you'd need to migrate either to SQL Server or MySQL, depending on the options offered by your host. Both are able to handle your needs.
That will be no problem as long as you run the mod setup for each of the mods.
It's very important to follow that strategy, otherwise your database will have structural problems that can compromise the way the forum works now and in the future.