The two pages I had you create should demonstrate the problem outside of eCart. There is no way the issue can be eCart or the php version.
You shouldn't be discussing eCart or anything else other than the two sample pages with the tech so that they don't get confused. The issue is that the session_save_path is set to a directory that either doesn't exist or have the proper permissions.
Only show the tech the two pages that demonstrate the problem in the fewest lines of code possible. They shouldn't be able to deflect it that way and claim it is an eCart or php issue.