When starting startplasmacompositor from gdm, sddm or tty user session, startplasmacompositor fails with the message "startplasmacompositor: Could not start D-Bus. Can you call qdbus?"
When trying to run qdbus independently, it says : "failed to connect to socket /run/user/0/bus : permission denied".

Reproducible: Always

Steps to Reproduce:
1. Start gdm / sddm
2. Try to connect with a Plasma wayland session

or

1. log in a tty user session (not root) and try to start startplasmacompositor
1.bis start qdbus its error

Actual Results:  
startplasmacompositor outputs
 "startplasmacompositor: Could not start D-Bus. Can you call qdbus?"
qdbus outputs
"failed to connect to socket /run/user/0/bus : permission denied".

Expected Results:  
Should start normally

It's a problem with dbus, so the error is similar to 357744
but in my case, the qdbus executable is found correctly but gets error starting