protools 2019(mac) unable to recognize protools 6 session

[carlstuck]

During the lockdown i decided to do some work at home on a song i started in a local studio so i dumped a wav file from my studio session into my old protools 6 rig, clicked the "enforce mac/windows compatiblility" box and started recording. Because i didn't want to create wav files of every take i recorded i just clicked "save copy" to a thumb drive and dropped it at the studio for my engineer to use but when he tried opening it his protools 2019 didn't recognize it as a protools session file.
To see if it was something i had done wrong i asked him to save a copy of our original session (with mac/pc compatibility enforced) and when i tried to open it, my computer opened protools but gave me an error dialog box that says "could not complete your request because: a program error occurred (-20006)". I'm not as concerned about not being able to open his session but i would like him to be able to open mine.
Is this something that has changed in pt or am i just missing something? I thought pt was always backward compatible.

When he saves from his mac the session file shows as .ptx and when i save from my pc the file appears as a .pts


My rig: (i know it's an antique but it still works) Carillon-AC1 pc running windows xp home 2002 home, 002 and presonus digimax fs interfaces.
10 gig system c drive, 70 gig D drive, 2x 80 gig music drives E & F

His rig: 2012 mac running high sierra with protools 2019.12

Any help or ideas would be greatly appreciated. Thank you in advance for your help. And i know i need to upgrade but as a full-time musician in the midst of a pandemic with almost zero income i just don't have the jack so i have to use this dinosaur til it goes extinct.