Occurred at CL 40095, which definitely touched a lot of the compiler. Frustratingly, there is no indication in the logs whatsoever what failed, and I don't see anything obviously racy in that CL. Running a bunch of race-enabled builds locally has yet to generate a failure. I'll keep at it.
That looks more like the builder's (my) fault than yours.
Maybe the new RPC just added for #20222 failed but something up the chain didn't handle the temporary error nicely.
In any case, those logs suck. I had assumed an RPC failure would result in a retry, but perhaps that's only for trybots. It should write more debug info the build log on failures, just in case it's not retried.