The main reason for this is when the database is open somewhere else.
This could be as already said by many others:
When recently I had the same problem although different versions, I started to search for the reason.
In my case I had an excel file that queried data from the database.
In case the excel file is opened before the database, access would give this error.
Conclusion:
Check all files/connections related to the database. An open ODBC connection or query used in excel (my case) open the mdb file and will make it readonly.