I wrote on another forum about my 'fun' doing a factory reset after the 6.01 update.
It was working fine after the initial OTA update but I usually to a factory reset anyway after a major update.
So after the factory reset, a couple of menus in and the reactivation lock kicked in, nothing new here as I've had that with the lollipop version 5.1, but this time after entering my Samsung account details all I got was 'process failed' (Google this, it's more common than you would think) so after endless retrys and a phone call to Samsung customer service, nothing changed!
So the only possible thing I could think of was to revert back to 5.1 using Odin, set phone up and then deactivate Samsung reactivation lock, then update to marshmallow again, all went to plan and it's now running sweet.
I hope this helps at least one person because it was a right pain and time consuming!