ObjectIsReadOnly on checkout

I have a program that checks out images and checks in images. It failed today with an exception:

CheckOut Exception [-102009] Unable to complete your request, you are not allowed to alter object "GUID-"...

A few minutes later, the same user doesn't experience this error.

Do you know what the error means? It's not one of the already checked out errors.

[edited by: Kendall Shaw at 7:55 PM (GMT 0) on 27 Feb 2023]