I found a solution to the problem!
I struggled with doing multi-label classification using the transformers Trainer until I found the following modification of the Trainer class here: Trainer — transformers 4.11.3 documentation 1

For some reason I also modified the Trainer for a multi class classification problem I have:

First, I discovered that there is no need for doing that, because the Trainer already does multi class by default 
The problem seemed to be caused by my multi-class Trainer modification