• Forge v28.1.115
• Snow Real Magic v1.5.2
• Kiwi v2.6.1
• Hwyla v1.10.6-B67
• Explorercraft v3.2.4
• Cube+ v1.1.2
• Have OptiFine installed? No

This crash is a bit complicated. It took me a while to figure out what was going on.

• The mod Cube+ seems to override stair block placement to place it's own equivalent of stairs, so with this mod, snow layers cannot be placed on those stairs.
• But if /setblock is used or if a stair from a structure is found, then its a normal Minecraft stair block. If a snow layer is placed on this stair block, then I get a crash with Hwyla when looking at the snowy stair block.
• And for some reason, Explorercraft has to be installed or there isn't a crash. And removing either Hwyla or Cube+ will also not give a crash.
  Crash Report
  Hwyla Error Output

I have no idea what is going on and I do not touch rendering things. Maybe you should report it to Explorercraft. Thx