oceldot, probably you have saved your project in folder and then saved in file, right? Ignore this user who is making more trouble than helping you and make the follow.
Change the name of your .CAPX to .ZIP, unzip it and search for the texture folder.
There will be some texture files, but, as the message is saying, it's missing one, called by "Particle.png".
If you have a backup of this file, restore it inside the texture folder, with the exactly name, even the lower and upper case.
Open your project inside this folder and you'll be able to work again...
This issue is common for me, happen in 2 of 10 times, while doing the save of a folder to a capx, and happen randomly when working with C2. Since R50 I did by myself one or two bug reports about this issue, being ignored, because it's not easy reproduced, and without proofs or solid reproduction steps, you can't help the developer to find what's wrong with the code.
I'm sure there is something wrong with the "Texture" saving system while porting a folder to CAPX, because the issue don't happen with other folders and files.