What this problem ended up being is that the Therion installation on the USB drive became corrupt somehow. The mpost executable disappeared. Reinstalling fixed the problem perfectly.
Well, this is another problem. It is probably due to installation on
USB drive. Therion on Win32 has path to metapost and pdftex written to
registry. If this error appears on your computer at home -
reinstallation of therion could help. If it is on therion from USB
drive - probably you need to set path to mpost/pdftex directly in
therion.ini file.
HTH, S.
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