1. Please describe as detailed as possible what you did and which problem that leads to.
When I tried to upload image with size: 4,16 MB (4 367 871 bytes) and resolution 2560 x 1440 as a preview image using the content tool's Workshop Upload Preview Image feature, I get the following error:
This error returns when you close it and it keeps repeating until I kill the contentool.exe
EDIT: Same happens when the same action is succesful (I made image smaller (filesize):