When my dealer ran that one, it went back to 20136. Ran rough and gave me a lot of weird indications, but it ran. But, they left the wifi/auto updates on (and I didn't check) and after a couple of days, 21020 showed up, along with all of the issues. I manually rolled back to 20282 but this time ensured that wifi/autoupdate are both OFF. Maps remained at NA 2.20. 20282 seems to run OK, if a little but slower than before this whole mess started.
I've had in car navs from Jaguar, Saab, Ram and SYNC 2 & 3 from Ford in previous cars. NEVER had a mess like this before. My previous Sync updates are seemed to work well and improved performance in places that were noticeable. But, clearly this one (21020) is really buggy and should NOT have been released without more testing!!
I expect Ford to fix it in a release sooner rather than later ... although I doubt you'll ever hear them say "Oops" or "Sorry"!!
Ken