I have just published a workaround on my blog, an accceptable and good solution until Milenko (the author of Mystique) fixes said bug.
http://www.byteproject.net/?p=56
Oh, I forgot: deactivating any of those plugins does not affect the bug at all. It is NOT a browser cache issue and my host is running a proper version of PHP5.
My website is http://www.byteproject.net and my twitter account is: byteproject
I have exactly the same problem, mentioning this since Mystique 1.4 but sadly the problem did not solve until present day. As workaround I implemented the standard widget from Twitter. Works but it’s ugly. Sometimes the Twitter timeline shows up, sometimes not, you will recognize the same on your page.

Actually running WP 3.0 Beta 1, Mystique theme 2.2.2 and the following plugins:
Contact Form 7, Find me on, Lightbox Plus, More Linke modifier, SI Captcha Anti-Spam, Syntax Highlighter Evolved, TinyMCE Advanced, WordPress Beta Tester, WordPress Media Flickr, WPTouch iPhone Theme.