I too, had the "Attempt to call a nil value stack traceback error".I would like to confirm that there is absolutely NO ERROR with Accu-Feel.
As some folks had suggested the error was due to a corrupt saved flight, I removed all recently saved flights.
Did not help.
Today, it occurred to me that I had uninstalled "Accu-Feel" a while back, using "Control Panel - Programs & Features" (I have Windows 7 64 Bit). However, Accu-Feel never did go away from the FSX toolbar under "addons".
I didn't think much of it, but I was puzzled as to why it didn't completely disappear from the toolbar.
Well today, I had that "Eureka" moment. I re-installed "Accu-Feel" (as well as the 1.091 update).
No more "nil value" error.
First of all the FSX Menu toolbar "addons" will show the "Accu-Feel" feature as long as it is included with your "dll.xml" file where all those add-on are registered like the FSX SDK Toolbox Menu.
Then under the FSX-MAIN Folder there must be the A2A Folder that has a subfolder "feel".
If you want to test/create the error advice just delete the folder "feel" (backup before).
I'm sure the error is not caused by Accu-Sim but with a wrong or corrupted installation.