This is such bizarre bug. So as far as compatibility with the OS goes, so long as the game runs the mod is compatible. I started the work on Windows 7 and am now on Windows 10.
Steam can be ****ey which is why the default install to ProgramFiles(x86) is terrible (which I think I've found out that that's Windows fault). Both entries in your Steam library will launch the 1.5 version IF you have the U-Boat Missions installed, it's all in one install folder anyways.
So my suggestion is to redownload the mod, perhaps the mod files became corrupted somehow.