Yup... frequent problem. Knife Edge has a support forum that may lead you completely through the fix. Most likely trouble is with one or more on-going Processes within the System that is unfamiliar to RealFlight. Like Web-shots, etc.

Right click the Task Bar and select 'Task Manager'. Select "Processes" and look at all the ongoing (or active) applications. Simply selecting those that are not identified in the User Name column as "System" and then clicking the "End Process" button for each possible conflicting app will usually allow RealFlight and its Add-Ons to then load properly after closing the Task manager. After Re-start, all those ended processes will return, but your installation will be good and RF runs well.

More info on Knife Edge's website, but this will likely get you going. Hth,