First off, trying both Chrome and Safari on High Sierra - your customer support form at https://symless.com/contact/customer-support does _not_ upload. Something about your attempts at validating form fields seems to be erroring out, blocking uploading the form. Here's the log files:
https://synergy-logs.symless.com/f1a580f90b38117d6a08effc2ee608c0/logs/67-2018-01-03T09-34-20.log and https://synergy-logs.symless.com/f1a580f90b38117d6a08effc2ee608c0/logs/67-2018-01-03T09-57-36.log
My suspicion is that your SSL certificate is broken, and folks running recent OS versions have SSL libraries that refuse to validate connections to sites that a) use a self signed certificate and/or b) have an ssl certificate that does _not_ match the hostname being connected to - see https://www.sslshopper.com/ssl-checker.html#hostname=pubsub1.cloud.symless.com - it's 2017, with services out there like Let's Encrypt, who self signs a public facing web service anymore? (And, for that matter, gets the hostname on the self signed certificate wrong?)
When I saw that Synergy 2 had come out of Beta, I had high hopes for being able to start using it again.