Plutonium

It's due to the (I believe) 2GB maximum texture size allocation. For me, it happened right after I made my own skinpack. By removing the skinpack, it loads fine. Could be different to your issue, but that is just mine anyhow.

this is happening to me just after I changed mods, it's probably something to do with mods (or adding custom files in general)

It didn't work when I installed it? I'm sure I installed it correctly. Maybe I did it wrong, I haven't been active here lately.

To install I went into Bo1 images folder in %localappdata%\plutonium... that didn't work. (rather I put the claymore.iwd in there )
Then I went into my Bo1 steam folder and found the .iwd which the claymore is stored in, unpacked, replaced, repacked, still didn't work.

Do I need a separate patch for this animated claymore to work, or no?

Very nice! Good work