Donjo, I posted this a while back as I like many others were having a problem opening doors on the default 737-800. Go into your menu for button controls, and look for Shift-e for opening doors. Deleat Shift-e and assign a new SINGLE key. It can be any single key you wish that does not conflict with anything else; I am using \ (backslash)
Open doors as follows:- Just pressing \ will open main exit forward door. \ + 2 will open cargo/hold doors. \ + 3 will open other doors for catering trucks etc. To close doors just repeat as above.