One item to double check is that if you have not blocked Kindle for PC from updating, it will have updated to 2.4.1 in the background. You will have to manually download K4PC 2.4.0 or earlier and re-install it over the current install. The Amazon link for 2.4.0 (2.4.70904) still works:
https://kindleforpc.s3.amazonaws.com...-2.4.70904.exe
There is a batch file that will create a file in the Kindle directory to block K4PC's ability to update. See
message #84 in this thread.
You might also check that you have installed the latest alpha release of NoDRM's fork of DeDRM found at
Autoreleases.