Comment # 7 on bug 1091818 from Neil Rickert

I am marking this as confirmed.

I logged into Plasma-wayland.  And then I ran a command that use gpg, so that
gpg-agent was started.

I then logged out, and logged back into Plasma-X.  And the session failed to
start.  So I killed the running gpg-agent, and then I was able to start the
X-session.

Here's my analysis of what is happening:

When running Plasma-X, "gpg-agent" is automatically started as part of X
startup.  And the X-session is a child of gpg-agent.  So when the X-session
exits, gpg-agent shuts down.

When running Plasma-Wayland, "gpg-agent" is not started.  It is, however,
started on demand when first used.  As such, it is independent of the Wayland
session and does not automatically shutdown when the Wayland session ends.  But
it blocks the startup of X because that fails already when starting gpg-agent.

Solution:  Both X and Wayland should be handled the same way.  Either both
should start gpg-agent and make the desktop session a child of that agent -- or
neither should start gpg-agent.  In either case you would not run into this
problem.

Temporary workaround: killing gpg-agent in
  "$HOME/.config/plasma-workspace/shutdown"
should work.  The man pages for gpg-agent indicate how to kill gpg-agent.