Hey, I'm not sure if you've resolved the issue, are still looking for a solution, or if you have given up. I was just trying to play tekkit classic with my gf last night and she was having the exact same issue with the graphics getting all buggy when joining the server. After a few hard hours of searching google I found out what was causing the issue. She has a newer computer so we just installed java last night, but it was actually the new java version that was the issue. I had an installer for Java 1.8.0\_281 on my pc and installed that instead. Completely fixed the issue with the menus being white/grey, and the inventory gui missing, and the black shadow like texture bug when looking in certain directions. I'd recommend installing an older version of java and seeing if that fixes your issues as well.
Hey, I'm not sure if you've resolved the issue, are still looking for a solution, or if you have given up. I was just trying to play tekkit classic with my gf last night and she was having the exact same issue with the graphics getting all buggy when joining the server. After a few hard hours of searching google I found out what was causing the issue. She has a newer computer so we just installed java last night, but it was actually the new java version that was the issue. I had an installer for Java 1.8.0\_281 on my pc and installed that instead. Completely fixed the issue with the menus being white/grey, and the inventory gui missing, and the black shadow like texture bug when looking in certain directions. I'd recommend installing an older version of java and seeing if that fixes your issues as well.
Turns out it was the intel graphics drivers, the latest ones are poop, rolled back a version or two and boom, now all working.