there, I'm having issues connecting two sites to each other

Hi there, I'm having issues connecting two sites to each other using the User Sync plugin.
I have two different domains, on two different servers.
One domain (Master) is https, the other (Sub) isn't.
I've enabled Debug Mode, but the log files are not being created in the /plugins/user-sync/log/ directory for either site.
Any help would be much appreciated.