OK - so we may not have moved far in finding out what the issue is, but we have ruled out a few things its not:
1. its not a theme issue
2. if the majority of your users are seeing the question but the select group are not, then it seems unlikely its an issue with the moodle installation itself, or the question itself, but that will probably be why you are not seeing any php errors - there are none (or it wouldn;t work for the majority of your users)
Is there something they all have in common (they are using one browser/version and everyone else is using something different OR they have a different version of flash installed, etc)?
Do they have a particular plugin enabled that others do not?
It does seem likely that it is something user specific as you say it works for the majority of your users, rather than something at the core of Moodle. But hopefully someone with a bit more experience in this side of error catching can jump in and advise, my main point was to check the other themes as you had a concern that the theme may be blocking the error reporting.