I solved this after having the same problem with BestSync 2018 Ultimate.
First I de-installed all further versions of BestSync, deleted remaining folders on HDD, checked and cleared all entries in registry database named BestSync and/or RiseFly.
Then I installed the newest version of the product and activated the key again.
From now on the software hasn't lost the activation and worked probably.
Don't forget to export your jobs before removing older versions.
Good luck and please keep me informed when I'm right.
Regards - Dieter