It is not a known issue, it doesn't even sound possible to me (not that I'm doubting it has happened for you) . The forum software (Xenforo) is very standard and is designed to work on any *modern browser and device.
What actually happens, do you get ana error notices. Screenshot of what happens would be useful. Also list any extensions and what OS and safari version. (Porobably worth clearing cache and cookies first though)
I will feed it back to the xenforo development team if it is something that can pinned down.
NB: I have heard of issues with IE on Windows 7 - but that is due to the higher spec server security we have that such obsolte systems cannot deal with - obviously not the case here
I'm running OS X Yosemite 10.10.5 and Safari 10.1.2. I've tried clearing the cache, cookies, disabling extensions, restarting my wifi, and resetting the DNS servers. It seems like an encryption issue? I'm also unable to see a certificate for the site in Keychain Access.
The reason I believe is because of the high level of security on our servers, some older browsers are not compatible. I did know of issues with IE on Windows 7 (no surprise - and I doubt anyone here is using that) however I was not aware that the same issue was there with older safari versions.
In the case of IE I asked the server host if they could reconfigure, and the response, understandably, was that they would not do that on shared hosting as it would compromise security (lowering of TLS / SSL / Ciphersuites levels). I can ask again in this case but I imagine the response will be the same. If we switch to dedicated hosting they would be prepared to lower the security, but this forum does not generate enough income to justify the expense.
Would it be possible for you to upgrade Safari.
Meanwhile I am on the case, still investigating ideal solutions....