If you open the top cover of feeder you'll observe on the bottom of the input tray a mylar ramp that goes to a square rubber. This is called the retard pad and is charge of stopping multiple sheets of paper from feeding.
This assembly is spring loaded and may need to be poped off and reseated. If there is no rubber the assembly will need to be replaced as it's function will be lost. You can also try cleaning this rubber pad with water (it can get slick with paper dust.
Paper typically sticks together due to humidity (multifeeds) the glue would be causing the jamming if it gets on the paper and makes the paper stick to something else.
I don't know where you put insulating tape, but if you make a warranty claim you will want to be very thorough removing any trace of it first.
Hello, 2 weeks ago I unpacked totally new C405, connected it and tested. Printing went well instead of scanning - every time there was paper jam in ADF, so I opened ADF cover and there was some kind of sticker which was blocking the paper. Also on the sticker there was a foilc handle suggesting to remove it, so I did and then everything worked well, but the sticker left some glue, it seemed strange to me, but was working so no problem. But after two weeks that glue got rubbed and made the paper jam again. I decided to clean that glue, after that ADF was working but taking even 3 pages at once - well, then I put a few layers of insulating tape on it and it works well again, but the question is how long, and in general - what the hell? :) Should I ask for guarantee service?