I've been a happy user of the /2.1/similar Stack Exchange API for a while. But, at some point within the last 24 hours, or so, it started returning empty results (but otherwise reporting everything as good).
I am seeing the same behavior from the doc examples.
It does not matter which site I search or which search terms I use. And yes; I triple checked that I am not busting the rate limit or otherwise getting errors reported :-)