In 1.8.9 there’s a strange bug with ConnectTextures that seems to only affact brown stained glass, which causes the client crash.
Further details:
As noted on the issue tracker, deleting these specific ctm files stops the crash from occurring. This is not an ideal solution if ctm is needed on this block - this is an issue for resourcepack devs to make a decision on before updating to 1.8.9
Not sure if this is anyone other than @wheellee’s domain - feel free to tag below
I have tested this with single player-it does not crash me there. It appears to happen with the server only, at least as far as I know. However, I can delete the CTM files and use the middle texture of the connected texture if it still causes problems-it’ll look essentially the same as it does now.
The crash is occurring while ConnectedTextures is checking neighbouring blocks - your single-player world probably just doesn’t have the same combinations of blocks next to each other that are causing this particular crash.
It’s not a server issue, it’s a bug with ConnectedTextures/Optifine - neither of which we can fix, so unfortunately it’ll have to be a work-around in the resourcepack - not so terrible in this case if it’ll look basically the same :]