Looks like the drive side is just a bit crooked from when you loosened it up & then retightened? And/or a bit too loose. Plus maybe the belt got worn & is not engaging fully like it should, compounded by not being exactly square, which is important.
Looking at the video again, is there a key on the shaft on the driven side, that could be where it's slipping.
(I do see a keyway on the driven shaft. Something to check.)
Last edited by oulufinn; 02/02/13.
