Hello thatadam,
What is the specific error that you are getting? There is one general thing to double check with an issue like this. It would be good to double check how your custom LUT was set up. For the camera to use the custom LUT it needs to meet specific settings, so if part of the LUT is not what the camera needs it would cause the issue you are seeing. Information on how the custom LUT file needs to be set up is in the Notes section on page 136 of the manual. If you need a copy of the current manual, it is available HERE. Once you are on the web page click on the Manuals button and the one to reference is named EOS C300 mark III EOS C500 Mark II Instruction Manual. I have also run across this as an issue. The LUT .cube file was a few megabytes too big for the camera to work with it.
