Icedove fails to open mailto: links

[Ralph Katz <ralph.katz@rcn.com>]

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Jessie, xfce, iceweasel preferred browser, icedove preferred mail
reader, Icedove fails to open mailto: links and iceweasel's Email Link
with the following error:

Icedove error message window, titled Close Icedove:
Icedove is already running, but is not responding. To open a new
window, you must first close the existing Icedove process, or restart
your system.

One instance of icedove is running.

This problem began several years ago with a thunderbird upgrade, and
it may come from an unknown legacy prefs setting or whatever that has
me stumped.  I've since dropped thunderbird for the icedove version.

Any suggestions for running this down?

Thanks,
Ralph
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