When connecting to an Access database on an IIS server where the site's worker process runs as 64-bit, the 32-bit JET driver will not function:
Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\path\to\access.mdb
Your connection string will need to be updated to use the 64-bit ACE driver:
Provider=Microsoft.ACE.OLEDB.12.0;Persist Security Info=False;Data Source=C:\path\to\access.mdb