I had a similar issue with the error. I've fixed my issue and got my bag 4 slots back by un-checking the bag to unequip it, then save, then open edit equipment again, drag (equip) the bag again and the slots and bag are back & working. So far.
ps
when I close out the Maxathon (edit correction 'Comodo IceDragon') browser, and then open it again, it seems the problem returns. I am now going to clear my data, again, and clear my cookies just to test it.
I did submit a bug report when it was glitching.