When upgrading from Login PI 3.4.x (or earlier) to 3.5.x there is a problem when accessing Portainer once the upgrade is completed. There is a simple way of fixing the issue. You need to change (or re-apply) the password of the virtual appliance.
To do this follow the following steps:
Log in to the Login PI 3 Virtual Appliance and Select "Change your Password" from the menu.
You are presented with the following window, here you need to type your new password. Note: It may be the same as the old one.
Once the password is filled in you need to confirm the password
When the you have confirmed the password the Virtual Appliance will reconfigure itself once this is done you can press enter and go back to the main menu.