[Edit] IDEA! I just tried to access the registry-tab, and the launcher said there was an error, cancelling, and then there was the tab. Could this be the one?
First, the list of command-lines: C:\Games\FreeSpace2\fs2_open_7-1-04.exe -spec -glow -pcx32 -jpgtga -ship_choice_3d -targetinfo -nomotiondebris -snd_preload -d3d_no_vsync -loadonlyused -dnoshowvid -fps -ambient_factor 0.39 -mod FLMusmod
Take that!

All should be tried and true (the noshowvid was added in case that was the problem). I removed all the options that were new to me for testing once (the texture and lighting-stuff was allowed to stay) - no luck.
VPs:
mv_core_a.vp
mv_effects.vp
mv_hiplanets.vp
mv_models.vp
mv_speffects.vp
mv_textures.vp
All standard FS2-VPs and sparkee.vp (insanely-high-res?)
No firewall that I can configure (if there's a firewall, it's most likely with my ISP). System specs:
ASUS P4T-E
Pentium 4 2.0 GHz
512 Mb RDRAM (PC2700?)
ATI Radeon 9700 Pro 128 Mb, Catalyst 4.4

Sound Blaster Live! Platinum 5.1
DVD and CD-R
Wingman Force 3D, software v4.40.
Proceeding to try a *sniff* stripped FS2_Open...
[Update] Uh-huh. It's something with the exe. The recent builds (26/6 and onwards) didn't look very lively, but 3.6 worked just fine with everything turned off. Will try turning everything back on...