Tech support got the forum up and running with some tweaks...
We got the forum loading with the following change to the functions/functions_common.asp file:
In the SSLswitch sub, this line:
strServerHttpsStatus = Request.ServerVariables("HTTPS")
Was replaced with:
If InStr(Request.ServerVariables("SERVER_NAME"),"ssl") = 0 Then
strServerHttpsStatus = "off"
Else
strServerHttpsStatus = "on"
End If
And I've replaced http:// in URLs with https:// so that browsers no longer show a warning of "connection not secure".