It may be possible that the user has some cached detail or an incorrect saved bookmark when they try to reach /tpam.
A couple things to try:
1. Logout, and then login directly to https://ipaddress/tpam
2. If the same 500 error is seen after entering the username/password and the address is not showing correctly as simply 'https://ipaddress/tpam/main.asp' then have the user remove the extra detail to see if the login has indeed been successful and then accessing the correct URL is successful.
3. Have the user test with a different browser or possibly 'InPrivate' browsing (in IE) or Incognito mode (if using Chrome)
I would expect this is being caused by either the browser incorrectly auto-completing an address or possibly a saved favorite with an incorrect address.
Clearing the browser cache/history might assist as well.
If this is just one user, I would not expect that this is related to TPAM but more likely the user's browser or possibly something with the VPN mangling the URL.
If the issue was affecting all users trying to access '/tpam' and receiving a 500 Internal Server error this might suggest the appliance needs a reboot. We have seen similar issues on appliances that have a very long uptime (and might require a reboot) however this would typically impact all users accessing '/tpam' and not just a single user.