I had this issue.

Definitely a lockring/cog (stripped thread on the hub) issue.

you can either
1. get a new hub/lockring and put red Locitite on the thread (you won't be able to remove cog/lockring after)

2. velosolo has a front mtb hub converted to fixie rear hub and sells cog with 6 holes so you can bolt in cogs to the hub (prolly best way if you skid alot)


i tried getting quality hub/cog/lock ring and had it properly installed but didn't really solve the problem for me, especially going uphill at slowspeed so lota pressure on crank or skidding on very grippy surface with grippy tyres

after stripping 2 hubs, i went with #2 option and never had a problem since.