Windows Live Mail could not be started because another instance of it is still running on your computer. etc.

Intermittently, about half the times I open Windows Live Mail, I get the following error message.

Windows Live Mail could not be started because another instance of it is still running on your computer.  Please log off Windows and then restart Windows Live Mail If the problem persists save your work and then restart your computer. 

This occurs even after a log out, reboot, or complete shut down and restart.
Checking, Task Manager, there is no other instance running.

It started when the new Windows 10 version was installed in December and shows no signs of repairing itself.
Initially in December 2017, a similar message appeared when shutting PC down stating that the other user may lose data, but that has been sorted and doesn't appear anymore.

Despite the messages, Windows Live Mail eventually starts and apparently runs normally.

What is causing this and how  do I stop it? It is annoying and worrying.