ellw ... THANKS! Super helpful and would have solved my issue even though I didn't use your link. Almost don't want to say this, but writing the post made me realize that I probably had an old installer on one of my machines even though I couldn't find it online ... and I did: 2.4.4. Wiped 2.5.6 and reinstalled local versions of 2.4.4 and immediately started syncing with my server instance. In retrospect, I believe this exact server/device version incompatibility issue happened to me a couple years ago (BT Sync days) and back then I re-arranged our workflow to take the rented server space out of the file sync equation. I understand the issue now and I'm going to be much slower on the update draw from here out.