Help needed in getting a cooperative mission to work

[GSHWK_Houndstone_Hawk]

Would someone be so kind as to check to see why on earth I cannot get this coop mission to work?! (Download link included).

I have 2 pc's, 2 Il-2 accounts, 2 copies of everything and have made coop missions (with media files) before with no hassles. I've done things no different this time around but whatever I do, change, & try, the connecting PC gets the dreaded #10017: filer transfer error.

I've checked that all media files etc in the .list file are pointed to the Multiplayer\Cooperative folder & all seems ok. 

I've tried deleting all content from both the Cooperative & Dogfight folders of the connecting (joining) pc but then I get the #10016: unable to save file to disk message because the media file won't upload (and when I manually insert the media file into the mission folder of the joining pc, I'm then back to the #10017 error.

 

I've also done (and redone) the save files & groups thing in the tools section of the mission editor. Yet the coop missions I made a while back (which include media files also), works as they always have done. So baffling and infuriating as we were destined for a bit of festive 'skip bombing' this weekend but this has got me stuck for 2 days or more. 

 

Any help from looking at the mission would be so very welcomed. It's a largish file I'm afraid (around 42mb & the only difference will be that I'm using custom A-20b skins whiuch I didn't include in the download as I'm guessing the game engine will just include generic skins.

 

Mission file is here:

 

https://www.dropbox.com/s/sg79uei0bi0n9o8/Practice_Mission-A-20B_Havoc.rar?dl=0

 

Many thanks in advance.