Currently, it is not possible to manage licenses via PTC "Manage Licenses" tool.
Solved! Go to Solution.
OK, it seems that PTCs tool does not like Firefox.
It works with MS Edge, but who uses MS Edge?
Ok, it seems, that they fixed it.
Since MS Edge is the default browser with Windows 10, probably a lot of people.
Yes, I use it almost exclusively for my daily Windchill interaction.