Finally tried to uninstall / reinstall as this was bugging me a lot (I always use my iPhone or Outlook on the PC to manage tasks - rarely TD.com).
It somehow fixed the issue. Just had to change a couple things in preferences afterwards.
As a matter of fact, I updated Toodledo on my iPhone on May 10... and soon after it stopped syncing "unable to sign on to Toodledo.com. Please go into your Synchronization Settings and check your email login and password. Error 1001"
All is fine with the login/password.
I have TD v 3.0.5 build 2602
I tried to force sync from iPhone to TD, and then the reverse, always the same error message.