Access the database by different users simultaneously
Since you mention that several users are working fine for a while, you have the database configured for multi-user Shared access.  

However, apparently someone on "machine no." is able to acquire Exclusive control of the database. The errors you are getting are the fallout from the Open Exclusive and likely does not indicate any corruption. 

This suggests that you also need to configure the security for the database to deny your users the Open Exclusive permission. 

There once was help on this at this at a now defunct link. For what it is worth, you can examine:  http://traycare.com/Help/NtwrkOps.htm  and see if it helps you.

EDIT: Note that changes to the database structure (Forms, Views, Tables, etc) require Open Exclusive and should be scheduled for a maintenance window so as to avoid unnecessarily interfering with users of the database.