'Microsoft.Xna.Framework.Content.ContentLoadExcept ion'

Since earlier this night, I've been getting this message when I try to load a certain level of my game, while testing it on my phone. The app simply freezes and this message pops up in MS VS.

'An exception of type 'Microsoft.Xna.Framework.Content.ContentLoadException' occurred in Microsoft.Xna.Framework.ni.dll but was not handled in user code'

I'm not a coder and can't really analyse the error. Googling it seems to suggest that it has something to do with the code trying to load the .png images wrong or something.
I've considered any changes I made to the file today, and made sure to re-load any .pngs used by my objects, just to be sure the problem doesn't occur because I've moved a couple of files around in my folder.

So what on earth could be causing this all of a sudden, and how do I get to the bottom of it?

Help greatly appreciated!

Edit:

Also of note is that the problem persists even if I use an earlier save of the project.

Quote from Francois

Can you send me your project folder, zipped, as well as the mfa?
Thanks...

I've sent a link in PM, thanks!

Quote from Francois

I'll only be able to work on it next week : I will be in Montpellier for the ClickJam the whole week end.

That's a shame. I have my presentations for this major project on Saturday, Monday, and Thursday.
Still, can't be helped. I appreciate that you will look into it then, at any rate. Thanks!

If anyone else has any clue of what's causing this to my game, please respond!

Update:

I've been trying to solve it without success, but might have pinned down the issue (although I'm not sure how to solve the issue).

The program always seems to break at this code:

string imgName = handle.ToString("D4");
imgName = "Img" + imgName;
image = app.content.Load<Texture2D>(imgName);

Furthermore, if I go under Content References in the Solution Explorer of Visual Studio, the [Gamename]Content tab is marked with an error symbol.

After googling around, this seems to suggest that there is an issue with loading the .pngs for my project. But I've gone through my objects in MMF and made sure to use .pngs of the correct file path in my updated textures folder for each one. No change.

The crash always occurs when I try to begin level 2 or 3 (with the click of a button in a level selection frame), and at the end of level 1, just as the 'Game Over / Victory' image object is about to appear. Not sure what the correlation could be there...

Does anyone have any ideas? I'm a bit desperate!