Somehow the patching got corrupted in the Research hook, and invalid IL code could not be processed. I don't know how this happened since I didn't change anything regarding research methods since a time, but whatever. It was made a bit better, and faster. Fougerite.dll, and the prepatched dlls / patcher was updated only.
PATCHER MODIFIED, REPATCH OR USE THE PREPATCHED DLLS.
Added an option in Fougerite.cfg 'EnableDefaultRustDecay', please add It. The reason why I added this option because I would like some people to research this. Back in those days I remember when I was playing on the official rust legacy servers, and they were lagging...