When both Krastorio and SE are installed, SE modifies some of K2's recipes (in this case Solid Rocket Fuel) in the post process mod. Current mod load order causes GDIW to reverse the original K2 recipes, then SE modifies the K2 recipe, leaving the reversed recipe no longer matching.

I was able to fix this locally by adding space-exploration-postprocess to the optional mod requirements list

How could I fix this locally myself, where do I make that change, it has managed to annoy me for ages.

Unzip the mod.
Open the file info.json in a text editor (notepad), add this line to the dependencies section

"(?) space-exploration-postprocess"

add a comma on the line above where you add that (just follow the pattern the other dependencies show).
I'd also suggest changing the version number just to make sure that your mod doesn't 'match' other people's accidentally if you play on servers. you can just set it to 2.1.100 or something. just different enough it will never match an official version.
Save the file
rezip the mod.
restart the game.

If you play on a server or with other people, they will also need the modified zip once you're done.