Crashing and Corruption

[LimitedTech]

I have 2 databases.
1 (Database A) was created in Access 2000 and has been in use about 18 years with no issues.
The other (Database B) was created about 1.5 years ago in 2003.
Both have been converted to Access 365 databases.
Both have a front end and a backend.
Both are used by max 5 users at any given time.
The backends run on a standard Windows 10 machine as the server.

They share 3 tables.
Records from Database B are exported to Database A at the end of the day.

The databases have started crashing. They give the "unrecognized database format" error.
They were crashing before they were converted to 365.
If I open the backend, it repairs itself and users can return and continue working. Repeating the action doesn't produce another crash / error message.
In database A the error normally occurs at the same time, when doing a specific action.
In database B crashes occur seemingly at random. This one has 3 users in it constantly. The crashes do not appear to be caused by users in the same records at once.

I have zero experience tracking down errors. Where is the best place to start? Is there a standard procedure for finding errors?

 

[kray4660]

You say you have a front end, but does each user have their own copy of the front end (it is a long shot since I know practically nothing about 365)?

 

[LimitedTech]

Yes each machine has a copy of the front end for both databases.

 

[kray4660]

I am not sure, it almost sounds like the connection between the front end (users computers) and back end has some sort of connection issues. I do not know how to diagnose.