One of the hosted management tools is the 1&1 VPS login which uses SSL to port 8443 to login. At one client site they're still using an old ISA 2004 firewall and proxy, which blocks this login.
Adding a rule to allow traffic to 8443 doesn't fix this, you need to allow SSL over port 8443.
There's a good note on isaserver.org, which is probably the best source for this product, however it refers to a download for isa_tpr.js from isatools that is no longer available. After a few dead-ends I found the isa_tpr.zip download on Trend Micro's website. You need to set an SSL port of 4343 for their Worry-Free Business Security (WFBS) console too.
Run these lines in the command prompt of your ISA 2004 server
cscript isa_tpr.js /add ssl8443 8443
cscript isa_tpr.js /show
Then restart the Microsoft Firewall service to apply.