Last report as I found a reliable hack for running experiments within a decent time-frame 

Set num_workers=0 in validation loader
Set reload_dataloaders_every_epoch=True (PL argument)

Cheers 
Acknowledgements: @ptrblck . Brais & Lukasz for listening and coming out with suggestions. Fuwen for suggestions and spending some time googling the error, and coming up with something non-trivial