Restarting the app or rebooting the device should be sufficient. What kind of devices do you actually synchronize, are incoming Bonjour connections enabled on all of them or only on one central server (as recommended)?
A screenshot showing the old & new sync location might be useful. In addition, please choose Help > Report Bug in DEVONthink 3 while pressing the Alt modifier key and send the result to cgrunenberg - at - devon-technologies.com - thanks!
I don’t if this is your case, but I have met this problem for some time and changing the sync cable by a new one seems to solve the problem. I could charge my ipad with the old one but maybe the sync part was having problems.