Can not open Gnome snaps as they are not connected to «gnome platform snap»

After upgrading Ubuntu from 17.10 to 18.04.1 I can no longer start gnome-system-monitor or any of the other snaps named «gnome-*» (that were installed automatically).

Trying to open System monitor using its icon, nothing happens apart from a spinning mouse pointer appearing for a few seconds.

Trying to run «gnome-system-monitor» from the terminal fails immediately with this output:
«
/snap/gnome-system-monitor/51/bin/desktop-launch: line 23: /home/mathias/.config/user-dirs.dirs: Permission denied
You need to connect this snap to the gnome platform snap.

You can do this with those commands:
snap install gnome-3-26-1604
snap connect gnome-system-monitor:gnome-3-26-1604 gnome-3-26-1604

(the '3-26-1604' number defines the platform version and might change)
»

This is the output from «snap list»:
«
Name Version Rev Tracking Publisher Notes
amparemarkdownreader 1.0.0 1 stable juthawong -
atom 1.28.2 185 stable snapcrafters classic
core 16-2.33.1 4917 stable canonical core
gitkraken 3.6.6 74 stable mmtrt -
gnome-3-26-1604 3.26.0 70 stable canonical -
gnome-calculator 3.28.2 180 stable/… canonical -
gnome-characters 3.28.2 103 stable/… canonical -
gnome-logs 3.28.2 37 stable/… canonical -
gnome-system-monitor 3.28.2 51 stable/… canonical -
gtk-common-themes 0.1 319 stable/… canonical -
signal-desktop 1.15.0 69 stable snapcrafters -
»

Running gnome-calculator, gnome-characters, gnome-logs or gnome-system-monitor using «snap run gnome-calculator» etc. gives similar results – with one exception: gnome-system-monitor is the only one that gives the «Permission denied» line quoted above. Interestingly, simply running the command «gnome-calculator» works. Maybe I have it installed both as a snap and as a normal application?

I tested following the instructions for gnome-system-monitor and gnome-logs, and that does indeed fix the problem for those snaps, and I can now launch System monitor normally. But there should of course be no need for me to fix this in the first place.

I filed this bug using «ubuntu-bug snapd» as it is obviously related to snap, but I am by no means sure that this is where the problem is. I have installed all available updates.

«snap version» gives the following output:
«
snap 2.34.2+18.04
snapd 2.34.2+18.04
series 16
ubuntu 18.04
kernel 4.15.0-29-generic
»

I am happy to provide more information to help get this issue fixed.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: snapd 2.34.2+18.04
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat Aug 4 19:48:56 2018
EcryptfsInUse: Yes
InstallationDate: Installed on 2016-05-01 (825 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: snapd
UpgradeStatus: Upgraded to bionic on 2018-07-23 (11 days ago)