I've been getting that message for years, across many differing versions of R8x. It doesn't tend to break anything, but I find restarting the console clears it up. I am currently on a fresh R81.10 server, with database migration from R80.30, and I have seen it once in the last two weeks.
Funnily enough I just got it myself. I was in the native console, viewing policies, and I clicked the "Users and Computers" tab, and up it popped!
Once the error is cleared I can then flip through the tabs, and it does not reappear.
Console version is R81.10 (81.10.9600.404).