Without any noticeable reason for this to happen, the production efficiency goes down to zero and is stuck there. It goes back to 100% after turning the production off and on using the Standby button, but rapidly goes down to 0% again.
I've found another bug report with the same issue, but from a month or two ago.
I'm still getting this bug and have to restart the gamesave/server in order to fix it. Really annoying having to restart ever so often to fix something so simple, yet important.