I spend few hours trying to find why my non-console snaps fail to run with “Failed to connect display” message. I found that was due to -nolisten local option which disables listening on abstract socket in Xorg display server. I think it can be documented somewhere for new snap users.

This is fixed in snapd 2.48+ : https://github.com/snapcore/snapd/pull/9530 Note that AppArmor 3.0.0 has regression that makes using non-abstract socket impossible which is fixed in AppArmor 3.0.1