Comment 5 for bug 1662400

I was able to hit this issue when U8 crashed while ubuntu-app-test was open. Leaving behind in:
/run/user/1000/systemd/transient/ubuntu-app-launch-application-legacy-ubuntu-app-test-.service

Once this was stuck there even relogging seems to be causing this issue. If you remove that file then it works again.

To reproduce this issue:
1) In U8 on a terminal start ubuntu-app-test with any application
2) Copy the /run/user/1000/systemd/transient/ubuntu-app-launch-application-legacy-ubuntu-app-test-.service somewhere
3) Relog into U8
4) Copy the *.service file back into /run/user/1000/systemd/transient/
5) Try to use ubuntu-app-test again from a terminal in U8

Then you'll hit this error.