I have just posted a similar question to this but very soon found a quick workaround which doesn't involve messing with Port 8447 on our routers (although it does need to be open in the webserver itself). I enabled WiFi Hotspot Tethering on my mobile phone and connected my PC to the internet...
The only page which I cannot access in Plesk is the Updates page. I have opened firewall port 8447 but to no avail and after a few seconds I receive this error:
(URL followed by...) PR_END_OF_FILE_ERROR
Can anyone advise me?