refer to files that are part of the game, not the ones that Ankor made (in externalmedia directory).
The part that goes wrong is that the mission (editor) uses a "\" in the path name, while it should be "/"
So find the Media MCU that has this reference and change the path name accordingly
For example in SYN Squad Challenge - Central Offensive, in group "objectives" you'll find a Media MCU named "Photo Recon Dialog".
Change the path "swf\photorecon.gfx" to "swf/photorecon.gfx"
Other Media mcu's there are ARTI_MEDIA, Arty Spotting dialog and another Photo Recon Dialog. And then most of these are double as there are two objectives.
There is more simpler, quick fix: Open the .list file and simply delete the lines
then save the file again.
Keep in mind, everytime you change and save the mission, these lines will return in the .list file! So the first method is more work, but more future proof...
To start a coop on a dedicated server you need at least two people. This is annoying; I already asked the devs if this can be changed but I never saw a response.