I've tried running avidemux 2.5.4 (under Ubuntu Precise) on the original .ts file & the .mpg file produced by using ProjectX followed by mplex - f 8. In both cases on an approx 4GB file, it gave a window displayed 'Could not open file'.
Any solutions (apart from using avidemux 2.6.1) which failed - I've posted separately for that.
if the .ts is not opening in Precise (12.04.1 LTS). Try with 2.5.6, if not opening with that, my guess would be the .ts you're trying to open is with (HD TV) video codec AVC or H264. You have to use 2.6.1 for these.
64-bit version can be found here: avidemux Cli/QT/GTK 2.6.1 64-bit deb download ubuntu 12.04 LTS (http://www.avidemux.org/smf/index.php/topic,11708.15.html)
Use QT and not Gtk.
2.5.6 you need to build yourself, can't produce distributables for these: Compiling Avidemux 2.5.x (http://www.avidemux.org/admWiki/doku.php?id=build:install_2.5)