Sorry, think it was my keyboard. Got this Lenovo thing and I must have pushed the weird button above the Esc key. Turned it off and now it's fine.

Seems to be specifically when I press Alt. I changed the binding to / and that doesn't stop player movement.

In T192501#2819893, @Jordi435 wrote:

I think it's just you. I don't have this problem and I also haven't heard anyone else about it.
I'm using free look mode all the time while I’m running and I can't imagine I could play without it...