My last two roasts I’ve had the 6864 error as well. On the last but one, I downgraded then upgraded the firmware (running 540) and had no problems.
This morning the machine booted, then about 30 seconds later I got the 6864. But the machine showed a countdown timer, starting somewhere around 20 (Maybe?). It alternated between Preheat Temp (235 in my case) and the countdown like this: 235 -14 235 -13 235 -12, etc. When it hit 0, the beeping stopped and I was able to preheat.
I have not opened the machine to look at the fan. (I replaced the IBTS assembly when I first got the machine as it was not getting power)
I religiously check fan speed (see parenthetical comment above) and indeed the fan speed starts below 4,000 - but then ramps up to between 11,000 and 15,000 - it is quite variable.
I’ll keep roasting, and try to collect more data on the next roast.