Captivate 6: Issue with clickbox opening URLs

Question

I've finally made the leap for an actual client project to 6.0 (usually only do client work on 5.5) and am gutted to have run into quite a perplexing issue. It's a bog standard piece of work.  Simple conversion of a client storyboard (all their ID) into a captivate module.  There's no audio.  It has a couple of widgets in it to allow playing of hosted video and drag and drop games and a pile of advanced action stuff, but nothing exceptional.   Client has no LMS, but is hosting it on a sharepoint.   We've set it up so you can only get to the screen the 'click to start next module' once you've completed the module (the client's request).
No issues at all in our development testing.  No issues in our small group user testing.
We've now gone to our final user testing (about 30 users in various locations worldwide) and four of them have the same issue:
- They click on the button at the end to launch the next module and nothing happens.
It's very frustrating for me because I can't replicate the issue at my end, and because of the way the contract is set up I'm not allowed on site, nor to contact users directly, so am having to trouble shoot via email with single point of contact middleman.
It is a simple click box, with an open URL function.     It works fine for me.  It works fine for 26 out of 30 users.  It point blank doesn't work for 4 out of 30.  
It's a SWF output to flashplayer 10.     All users have IE7+ and Flashplayer 11+
I've rebuilt the clickbox slide from scratch once.  It is on top.  It works for me.   It still doesn't work for this small number of users.  
Undertandably, the client is reluctant to go-live to 800+ global users with that kind of test fail rate.  (Weirdly all the risky stuff like the video and the drag and drop works fine on the affected users!)
Yikes!  Any ideas?

Answer 1

Hi Rod, thanks for the reply.  Like I say in the post because of the way the contract is set up (I'm arms-length subcontracting for the physical training provider) I'm not allowed on client site, nor to contact users directly, so am having to trouble shoot via email with single-point-of-contact middleman.)  The arrangement is every bit as frustrating as it sounds, but is what it is.
So I am taking their word for it. 
That said, I've no reason to doubt them as the same issue has been reported independently from different physical locations, and all the test users will have managed to find their way sucessfully through far more complex functionality before they get stuck at that point.
I know that from time to time when you run flash locally you can have problems launching URLs unless local flash player settings are updated to allow it - is there any network level equivalent that a company (or even local machine) may have in place that could cause the same behaviour?   Or am I barking up the wrong tree?