OK...this is embarrasing! ...I started to go through the script line by line and guess what! ...the %FONTSDIR% has never been assigned any value at all! At least not now. In addition all the ocr files have been copied twice; both to the %SB_INSTALLDIR% and to the %FONTSDIR%. So to conclude: I assigned %FONTSDIR% = [FONTS] and "problem" solved!

The only remaining question is why the messagebox hasn't been shown when, for instance, installing on a regular Windows 7 machine. Life is a mystery!