The latest Windows 64 version (today, april, 12, both zip and installer versions) still claims for aften.exe. My dirty workaround was to copy aften.exe into the Avidemux folder. I used the true 64-bit one, but any Windows PE (32 or 64 bits) named aften.exe is accepted.
BTW, I noticed that the blue rectangle showing the part that will be cut does not always fit the cursor's position. The cutting point is several frames before the cursor position.