lately getting "unrecognized format" on the BE file.

It could be caused by a network error.
Are any workstations connected via WiFi? If so, wire them up.

Have you checked out these possible causes?
https://www.stellarinfo.com/blog/how-to-fix-microsoft-unrecognized-database-format-error/

I had this case when i had a new form that performed some "unusual" operations...like forcing to insert some values to a recordset....refactored it and problem went away...try to remember if you introduced some new functionality.

Hi,

Are any workstations connected via WiFi?

No.

try to remember if you introduced some new functionality.

Nothing that I know of.

@Mark,
will check that & post back.

Any other suggestions?

Thanks,
Ben

@Ben

I've been wrestling with this at a non-profit site for the past six months.  They do peer-to-peer sharing, instead of having a designated Windows server or NAS (or similar).  My next task is to dive into some very old code that seems to precipitate the problem.

and that BE is not an accde correct?   There is one other bug out there, which is when an older version of Access attempts to open a .accde saved with A2016.

Jim.

Depending on the sharing architecture used, here are some other possibilities to look at:

If the front-end (fe) file is used by more than one user at a time, the version of the first user to open the file wins!  
If a user opens the fe with, say, Access version 2010, then only users with Access 2010 will be able to open the file.
All other versions will produce errors such as the one your seeing trying to open the file.
Make SURE EVERYONE trying to use a shared fe file is using the SAME version of Access.

This may seem trivial, but did you compact & repair the BE?

Hi Experts,

They do peer-to-peer sharing, instead of having a designated Windows server or NAS (or similar).

No, this is not the case here.

If the front-end (fe) file is used by more than one user at a time

All users have separate FE, and all currently using 2007.

did you compact & repair the BE?

Yes.

and that BE is not an accde correct?   There is one other bug out there, which is when an older version of Access attempts to open a .accde saved with A2016.

It is, so very likely this is the culprit...as we had one user with 2016, and I was doing some changes over-there...
Guess will have to import everything (FE & BE) into new db using 2007.

Thanks,
Ben

<<It is, so very likely this is the culprit...as we had one user with 2016, and I was doing some changes over-there...>>

  That's a surprise; why bother?  If it's truly a back end with data tables only, then there is no need to make it an .accde

Jim.

Actually its not, my mistake, thought you were asking if its accdb or MDB...
So guess this is not the reason of corruption.

You should turn off the leasing and see if it clears things up.

Where do I do that, under control panel?
also is this to be done on the server or each user?

Thanks,
Ben

Would be done on the machine holding the BE.

Jim.

Hi,
Will not have have a chance to test that till Thu.
Will keep you posted.
Thanks,
Ben

Hi Jim,
The message we get is different from what that site mentions

"Microsoft Access has detected that this database is in an inconsistent state, and will attempt to recover the database."

Is this still applicable?
Thanks,
Ben

Potentially yes.  

 Also any recent changes in the network, software (anti-viurs), etc.  

Jim.

Also any recent changes in the network

Wasn't there long time, but according to users, no.
fyi- this is lately happening very often several times a day, and usually just by adding data.
Used to be a certain refresh combobox with a lot of data (multiple union queries combined) that was causing it, but they have stopped using that and still happening.
however if only one user is connected it does not happen.
Thanks,
Ben

<<however if only one user is connected it does not happen.>>

 *Definitely* disable the leasing.....that specifically is a feature related to more than one user sharing a file.

Jim.

So far it seems like this solved the problem.
Thank you!

Great!

Keep us updated.

Thanks,
Jim.

Hi Experts,
Just confirming that since applied Jim's suggestion, users didn't encounter this problem again.
Thanks,
Ben

Ben,

 Thanks for posting and following up.   Lots of people keeping an eye on this problem and this helps.

Jim.

Same here (all good) from a non-profit that I tend.

Thanks.

Jim.

I had this problem too, but I found some solutions to fix this problem. have you checked those solutions?
https://bigfactreview.com/unrecognized-database-format/