With Things installed, Immersive Portals see-through portals fail to render when a shader pack is enabled.

Mod list as per log file (after removing Things): https://pastebin.com/gFxHNHDK

Could you actually provide the log file? It should contain a lot more useful info, like the crash report, errors, etc.

Sorry this took a bit to get to. I've been swamped at work. I've just removed all mods except Sodium, Indium, Iris, Immersive Portals, Portal Guns, and Things, with all dependencies.

I've got screenshots showing shaders off, then on, and log file to match (enabling the shader threw a bunch of render thread related stuff, that's not relevant to this AFAIK). This was done in single player creative so as to not bork my server.

latest.log