This problem has driven me nuts for months. For a lucky coincidence, I had the SLV2 mod installed on my old laptop, so I never noticed this bug before.This goes for any StrangeLove hud whether it be SLV2, SLV5 or any of the SLV6 updates (RocketX).
If what you see while riding looks like the image below, then you are missing the Fonts Patch.
Once I started playing on my new laptop, each time I joined a server running those SLV2/RocketX jets, the HUD was blank like the one in the pic. The problem occurs also in certain MH maps, since they use RocketX jets as well.
I thought it had something to do with the newer video drivers, the custom UT renderers and stuff, but nothing seemed to work. Until I found this.