Just this warning doesn't really tell you much right away. The first thing to do is to figure out where it is attempting to retrieve this object (some data view, some microflow, etc.) and then try to determine if it's really not there and why the application seems to think it should be there. You can find out what possible ways there are for that object to be deleted and see if it makes sense that one of them was triggered.
Edit: If this is on creation of an object in a microflow, maybe you could try to set the 'Core' log node to trace and see if the object that was just created accidentally gets garbage collected. Can you also give some more information on your application specifics, for example if persistent sessions are enabled and what database you're using?
We had a similair problem with this. We also used a process where the user was guided through a couple of steps. Each step is a form. So the user can press back to go to form 1, press next to go to form 2 and a cancel button. The problem would arise with the cancel button. Because here we would open a new form where the user could choose to save the object before leaving, return to the object or delete the object. When the user choose delete we would see the same error that the object could not be retrieved.
My conclusions where that this error was generated because we did not close a form where this object was used. We choose not to close the form with this object before opening the cancel form because if the user would like to return to the object we would never know from which form the user would come since we have a lot of these type of forms.
I am still not sure if the error should be given though. At the other hand how would Mendix know that the form could be garbage collected?
Hope this helps to finding the root cause.
Regards,
Ronald