This happened to me too, only around Dec 22th 2084. Actually, I just noticed there's a remedy...
Hit Esc as soon as the "thank you for playing" screen comes. Seemed to work for me anyway (after 1 minute of testing).
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***PLOT SPOILERS BELOW!***
Just in case this info helps... Last mission involved "aliens trying to spread xvi virus". It was first mission of that kind. The map I got was 'corrupt_city04n' (in which one of my soldiers also got stuck on a ramp inside the huge ufo). The target location showed as TODO on Geoscape. The mission started between Dec 8th-12th, I think.
***SPOILERS END***
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On a related note, I fast forwarded few months after the "thanks" screen and got no missions. No ufos, no terror sites, no alien activity whatsoever. Also never got the technology that was supposed to lead to researching different UFO types, even though I had captured a fighter and a harvester. And they were even listed in the research list but the game wouldn't allow researching them. (although I heard capturing an intact ufo scout is a workaround for that)
Aaanyway... just hit Esc
(playing version 2.2, WinXP SP2 (updates from Jan 2008),... might edit in system specs later, too tired now)