I've been encountering this bug on various private testing missions, but until now, I've never nailed it on a released, otherwise working campaign. Now I found it on DE.
The build is the latest on from BP SVN, but this bug was there for quite some time.
It occurs when you go into a mission, quit it using the pause menu, then reenter it and try to go to either ship or weapon loadout selection.
I don't know when it first appeared, but I remember isolated instances happening a really long time ago. I suspect it's related to the way mission saves the loadout the player selected, since making even a minor edit to the mission (thus resetting the loadout) also "resets" it.
It doesn't matter if the mission in question is the part of the campaign. It also should be noted that this bug is not DE-exclusive, it's just the first released campaign I've managed to consistently replicate it on, as it doesn't seem to appear everywhere.
Here's the error message I've got:
Invalid model number -1 requested. Please post the call stack where an SCP coder can see it.
ntdll.dll! KiFastSystemCallRet
kernel32.dll! WaitForSingleObject + 18 bytes
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
fs2_open_3_6_13d_SSE2_BP.exe! <no symbol>
kernel32.dll! BaseThreadInitThunk + 18 bytes
ntdll.dll! RtlInitializeExceptionChain + 99 bytes
ntdll.dll! RtlInitializeExceptionChain + 54 bytes
Log is in the attachments.
[attachment deleted by a ninja]