Filament spool issues won't cause layer shifts in bowden printers, it's a risk only with direct drive.
At the root of it, there are only two possible causes for a layer shift like this - either the motor skipped steps or the belt slipped on the drive pulley, and judging by this gap, my guess would be the belt slipping because it's not tensioned properly - and even if it is, you should seat that belt correctly anyway.