MsAccess error 3046: "could not save; currently locked by another user"

The last couple of weeks (since the 6th of december, 2019) we get this error. 
We work with a MsAccess application in a multiple user environment (split database).
The program (frontend) did not not change. Also the server where the backend resides did not change and the way of working with the application did not change either. And now every day this message pops up and the proces stops.
I am sure there is no one else blocking the work to be done and every day it is a different set of records being processed.
The only way to proces the working set is to have everyone stop using the application and perform the task as the one and only person in the database.

Does anyone experience the same problem? And if so, do you know what is causing it?

Hoping for an answer,

Regards,
Ron