I was having this issue tonight as well. when i first start playing its usually not a problem, but after a couple of hours I start to lock up when trying to walk or sprint. i figured out that if I press another key while holding down the sprint button it 'unlocks' you. kinda weird. so basically double tap 'w' then quickly press and release 'a' while still holding 'w' and it would unlock for me. definitely a pain but once i got use to it, its not too bad. i wish i knew why it happened in the first place, as it seems fine when i first start playing, but thats alpha for you. ps-sorry for semi necro was searching interwebs to see if any others had the same issue as me.
