Yes that is all correct!
Just to add another observation about the GPU behaviour: I re-attempted to run a network with smaller batch-size (even though this had not worked before). Moving down from 64 to 16, surprisingly enough, the error did not arise, and the training process got under way. I then terminated the process, restarted the kernel, and tried again (still with batch-size 16 and the same model), and the error immediately arose.
Don’t know if that is helpful, but in any case thank you very much for all your time and help!