Ah thanks for getting back to me, was starting to wonder if you'd just dropped that as a note then disappeared after solving it yourself xD
Fullscreen Cogmind is fine on a multidisplay setup (I use two as well) as long as the primary monitor is the one with higher/equal resolution, which sounds like the case in your setup. So that shouldn't be it (unless you're intentionally trying to run it on the other monitor, which would crash, yes).
The fact that settings and saved games are working correctly should mean that it's not sandboxed, so that shouldn't be it, either... (that's the usual answer here, as mentioned in the readme troubleshooting guide
)
Not working in specifically true fullscreen mode is interesting... That said, there are no benefits to running Cogmind in that mode, it's just used by default since that's what people expect, but in general a lot of people switch it over to windowed fullscreen anyway since it's more convenient (certainly for multi-display players!).
Not sure what to make of this since we haven't had any other reports like it to crossreference with :/
Especially strange is that according to the log you posted it clearly chose the proper fonts and knows it's targeting a 1200p screen...
Just to be sure: You're not trying to run it fullscreen on the smaller monitor, are you?