Sync Server Problem (with solution)
 

The Omni Sync Server update that users were informed of via e-mail apparently caused my account to not be recognized when I tried syncing OmniFocus on all my devices.

Apparently, case sensitivity has been juggled around. Where I had been logging in with a username that contained capital letters, I had to change my username in the sync preferences to an all lower case version of the username. Syncing resumed as I changed each device over.

I will take note that the server update notice had my username in all lower case, so as long as your entered user name matches exactly how it was shown in that e-mail, you should be fine.