Bosch Exxcel 1600?

An error of F04 appeared indicating a blockage. I know that I need to undo the circular cap at the front of machine on the botton right. However, this appears to be stuck and will only turn a few degrees either way and will certainly not come out when pulled. How can I remove this cap to check what is blocking the machine? I am nervous to put too much force on it and break the plastic.
Hoping that someone can help....

Emma
February 2009