Avidemux "too short" error
That is way out of the h264 specs. According to Adobe After Effects the format constrains for h264 are at min. 10fps so even 2fps are (not, see below) out of spec and could result in issues with some players. So Avidemux seems to allow out of spec settings, that 1 fps isn't possible, is very likely an internal issue with how h264 gets encoded in Avidemux.

Edit: It seems this is indeed an Adobe only constrain and or constrain for MP4, when using the QuickTime format with h264 I can render with 1 FPS.

Still, I recommend you interpolate your video to be longer instead of decreasing the FPS unless you have very specific technical reasons for encoding with 1 FPS.
Showing every frame multiple times per second instead of just once per second doesn't make a visual difference.