After splitting a database into a front and backend the database operates ok
on a network for the rest of the day, however the following day it returns
the error unrecognised database format, and does not open.

andymerr46 wrote:
Compact and Repair. If that works, then create a new DB and import all
your old objects into it, then compact/repair again. That *might* fix
it. Sounds like you may have network issues. See Tony Toews's site
about that.