Problems with textures and more Thread

So I'm playing the hell on earth starter pack using ultimate freedoom and zandronum, which I started up by dragging both the hell on earth wad and the brutal doom pk3 file onto the zandronum.exe app. Throughout the game I noticed that the textures on some surfaces were missing, but I ignored them because they didn't affect the gameplay that much. However, right after the portal testing labs on the first level on earth, not only were almost ALL wall and floor textures missing, but there wasn't a single enemy throughout the level. Can someone tell me what I did wrong? And is there any way to fix this without uninstalling/reinstalling the mod, because I'm not an experienced old-school fps player (first time playing any doom game/mod) and most of the levels took me quite a while to complete.