I've encountered similar with OllyDbg. In the case of OllyDbg, when I loaded a LevelDLL, it first loaded a DllHost.exe, which was basically just a dummy executable used to create the process and address space to host the DLL being debugged. The problem was, the dummy EXE was at the preferred (and MSVC default) base address used by Outpost2.exe. When the levels made references to Outpost2.exe (as they do to call the exported methods used to interact with the game), it would cause Outpost2.exe to be loaded into the address space. However, the preferred base address was already taken, so it was relocated to another address.
The problem with NetFix, and other OP2Internal (formerly ForcedExports) projects, is they relied on having a known fixed base address for Outpost2.exe. Or rather, they relied on having a fixed relative offset between the Outpost2.exe module's base address and it's own module's base address. The two could be relocated independently. In NetFix, there is a safety check that causes it to abort loading with a failure code if the Outpost2.exe module is not loaded at its expected address.
I'm guessing something similar happened with the debugger you are using.
The workaround I used, was to have the debugger load Outpost2.exe first, and then load the level and debug it that way.
More long term, I'd like to change how OP2Internal works so modules can be independently relocated (which is how Windows works), rather than assume nothing will ever be relocated.