I have the same exact issue and it seems cirtexhosting is keep ingoring it maybe they dont want to admit that its server issue and not script issue,
their finaly answer was to reisntall script which I think will not make any difference, (I have cleared a template cache ) anyways so far I am not able to upload file bigger than 100 MB, it takes me to login page it seems that user session timesout, I hav session timout set to pretty high rate in htaccess though, not sure whats happaning,
I think best solution is to add a ftp upload feature .