Hi, GilB57.
I've tested with 'quick mission' and 'full mission' builder and everything seems okay over here. 'Single missions' seem okay too mostly. I do get some errors with 'Single Missions'(Bf109G-6Late/"String Index out of range:4"???), but that happens with or without this mod so I don't think it's the cause in that situation. The only other thing I can think of is renaming it so it loads first. As far as 'Front Markers go', I have no idea what that's about! I'm stumped here. Everything else in the mod is .mat, .tga's, and .eff's, so I don't see why those files should cause problems with loading. Can you access the log file from the mission loading failed screen? I've forgotten what version of Plutonium I started with(v1.3?), but could it be that there's a class file conflict with one of the other class files in another mod you're using. I would do a search for, gulp, every class file in the mod to see if that could be it. Sorry.
Pondering,
Steppenwolf