Hi,
for Windows:
I solved this problem by uninstalling Synergy.
I also had to delete the Synergy folders in C:\ProgramData\Synergy, C:\User\***your user***\AppData\Roaming\Synergy and C:\Progam Files\Synergy. (first 2 are hidden folders)
Somehow an older version saved the former IP Address in one of the config files. Nebst 127.0.0.1 there was also the old IP Address of my computer stored. (e.g.: locale.json in ProgramData\config\)
After reboot and new install it worked.
wbr