We had this problem with our C8145: our Tray 2 is set for "Tabloid 11 x 17" but every time we open the drawer it would say that the tray contained "A3 11.7 x 16.5". I went through the various tray settings, did a software reset, and the problem persisted.
Finally I removed the paper from the tray, moved the papr guides to their smallest setting, then placed them back at 11" and 17". That immediately fixed the problem: the tray now always comes up as "Tabloid 11 x 17". It looks like the paper guide sensors sometimes need a physical reset!
When I set it as a dedicated tray for 11x17 paper, it tells me there isn't 11x17 paper in it, but there is. I guess I'll have to call support.
Where you have it Fully Adjustable, it sounds like the Tray Guides are somehow getting out of position causing the loading error. You could see if you get better results as making it a dedicated tray. If not, you may need service call to check the tray sensors.
Our xerox machine has a dedicated paper tray for 11x17 paper, but every once in a while it will say there is not 11x17 paper in it. I have to log into it and set the paper management properties for tray content so it is "fully adjustable" and then manually put in 11x17 as the paper size. Is there something that can be done for this? For example, the machine is telling me there is JIS B4 (257 x 364 mm) paper in the 11x17 tray when I know for a fact it is 11x17 paper in the tray.