"One of your other computers is currently synchronizing. Please wait 2 minutes and try again."What can you do? (How can you solve this? What's the solution?) If you are encountering this problem and for the same reasons as it arose on my system, it is one that is, thankfully, relatively easy to fix.
I've seen several places online where people have asked about this bug, including http://lists.beatniksoftware.com/pipermail/tomboy-list-beatniksoftware.com/2010-July/001742.html and http://gnome-tomboy.1788872.n4.nabble.com/syncronization-problem-td1790091.html, and happily the latter forum includes the solution, provided by user Sandy Armstrong-2:
In the folder where you're syncing your notes, delete the "lock" file.
This should fix it.