I tried using a 800mm

Hey there derf69, the error code "err 01" indicates the need to clean the lens contacts, which in your case you have several. I suggest you completely clean the contacts using a contact cleaning solution which dissappears completely. you can also use a clean eraser to effectively remove any resilient buildups. I suggest you completely disassemble your lens configuration, and rebuild, once all contacts on the lens and camera body have been cleaned. There is also a possibility that you may have an incompatibility problem. If the cleaning does not help, please let me know and provide the exact model name and number of your camera, lens, and converter. Good Luck