Just posting this in case it helps anyone.
Syncing Thunderbird addressbook with Android stopped for me with the TB 68 upgrade and it wasn't this issue in the FAQ:
https://www.fjsoft.at/forum/viewtopic.php?t=31934
Turned out the Thunderbirthdays plugin stopped working with 68 and that was blocking sync. MPE was set to sync the calendar created by Thunderbirthdays but couldn't. Unchecked this calendar at File > Settings > Sync > Events and Tasks > Advanced
Then the other calendars synced OK again.