I was told I requested the sign-up code too fast. IMHO they’re receiving too many signups at once and they’re being rate-limited by their SMS provider…
Try insync (one-time payment), and works like a charm. I've been using it for a couple of years, zero problems. Syncs noy only OneDrive, also Google Drive and others.
They work already! When I add my email account, Thunderbird automatically configure my email, my calendar, my contacts, my tasks... all from my server. I even don't have to configure them each one manually.
Really? Wow! Ive never been able to get everything from fastmail to come over correctly. My calendars were read only and I didnt get full contact info.