Can't sign into iCloud on Mojave 10.14.6; no documented solutions are working

I can sign into iCloud on my iPhone, iPad, and even the appstore on my Mac. But not through the Mac system preferences. Not the iCloud preferences nor Internet Accounts. I type in my Apple ID and (correct) password, and after watching a spinning loading icon for a few seconds, I'm brought back to an empty Apple ID field, with no error message.

I tried:

sudo -v ; killall -9 accountsd com.apple.iCloudHelper ; defaults delete MobileMeAccounts ; mkdir ~/Library/Accounts/Backup ; mv ~/Library/Accounts/*.sqlite* ~/Library/Accounts/Backup/ ; killall -9 accountsd com.apple.iCloudHelper ; sudo reboot

which did not work. I tried temporarily moving

~/Library/Accounts/Application\ Support/iCloud

and the system seems to never generate any new contents.

I also tried the answer here:

https://apple.stackexchange.com/questions/338806/re-upgrade-to-macos-mojave-esp-parts-relating-to-icloud-without-loosing-data/341371#341371

I also tried creating a new admin user and logging into iCloud from there; this does not work either.\

Any suggestions?