UPDATE:
Everything below the line is applicable but here's an update and important new behavior:

After going through all the recommended approaches to fix this problem, nothing fixed the issue. Just to see if I was having any system or compatibility problems affecting the Eden Editor, I simply began a new SESSION. Eden starts up just fine and I can use the editor as I have always used it. To test if the Editor/Steam was behaving normally, I saved the new session/mission, exited ARMA, and then re-started ARMA to see if this new SESSION Mission could be started in EDEN. Recall that all my other missions CRASH the Eden editor when I attempt to edit them now. However, with this newly created file that I just saved will open just fine in EDEN and EDEN does not crash. Shockingly, EDEN crashes if I attempt to edit my older Malden mission builds! Whazzup with that? I noticed a Steam auto-update but I don't think it's related since I'm not seeing anyone else with this issue. I filed my Feedback report file. I hope this supplemental information brings some perspective. Obviously, my system and ARMA/EDEN are working but why EDEN rejects previously created and saved versions is a huge mystery. I use the editor literally hours and hours every day and I have tons of files that I have created. At this point, I can't load and edit any of them because EDEN crashes about 5 seconds after the opening EDEN map graphic is displayed. By the way, the map looks like it's going to be fine and I see all the evidence of my builds but I hold my breath for 5 seconds hoping it won't crash but it does. Now it doesn't even bother to throw the popup of the error exception. It just crashes back to STEAM. Hope this helps.

Below this line is the original description of my Eden issue. Thanks. I think it's obvious that I have STEAM and ARMA and EDEN working since I can use it as I always could but EDEN will simply crash if I try to continue editing projects that have been ongoing for weeks now. Arrrrgg......

I have a GTX 780 Video card (drivers are up to date) and was re-configuring my monitor resolutions etc, and when I subsequently started Steam and Arma 3 all is well until I try using the Eden Editor and resume editing my latest mission build. I can load and edit older mission files but not the latest 2 versions of my build. I can play the missions fine but cannot edit these latest two that I saved. I can play them but not edit them in EDEN. If I try to edit these "builds", the editor crashes immediately following the display of the opening map overview image. In that image, before EDEN fails, I can see that my mission is intact. But for some reason, Eden crashes at that point. I assume that the "Feedback Report File" has the error code for you since I don't have the exact verbiage of the popup placard. Sorry. I went back and reversed all the changes to my monitor's / NVIDIA settings but no help. Note that all I changes was "resolution" combinations, i.e., changing 1280x10xx to 19xx X xxxx, etc. I DID NOT change the in-game performance settings that I have used for months and months with this card and which have been working fine. I was just trying to see if my monitor could display clearer text characters in a different resolution. But, unfortunately, EDEN now crashes on my "builds" when I try loading them for editing. I do save these files often and use a progressive file naming scheme so that I can go to previous files. This crash happens on the last two versions of this file but not on versions previous to that. Still, I would like these last two versions to work in EDEN because they represent well over 15 hours of my editing time. Thank you for the help. I really need you to figure this out since it's well over my capability to even know where to start. Ciao. BTW: I always generate a "Feedback Report" files so I have the latest uploaded to you.