Thanks for the report. I've seen a few complaints about this issue on Mac and Linux, and I suspect it is an uninitialized buffer as you said. Unfortunately I'll be away from my dev machines for the next few days, but I'll take a look at this as soon as I'm able.
Small update here: I've been able to repro this on a non-dev machine and can confirm it's related to garbage data in a render target texture. A short-term workaround is to open the console and enter "set motionblurintensity 0". I'll continue investigating a proper fix as I'm able.