Hi @fmichielssen
This is the steps to reproduce the bug:
1. open two different sessions in two browser windows one normal, one incognito
2. browse to the shiny-proxy demo app on both session and login the same user, for example jack
3. run the hello-world app in one session
4. run the same hellop-world app in the other session
5. you will get the "Not authorized to access this proxy" error
So does indeed seem to be a bug.
I have submitted a bug report on github.