I’m also having the same issue with UE 5.0.3 and am unable to import any FBX animations.
I was using the exact same animation with the exact same settings in UE 5.0.2 and had no issues at all.
I did some digging in the source code of UE for to see what changed between 5.0.2 and 5.0.3 and it looks like a few lines of code with an additional error check have been implemented and are causing this issue.
This is the commit that added these lines (You need to have your github account connected to epic games to be able to see this):
https://github.com/EpicGames/UnrealEngine/commit/6cc65bdab13b31e33623314f39fee19972efc788
I just sent a bug report to epic games, but it will most likely take some time for this to be fixed so I’m urgently looking for a workaround.
Setting Animation Length to ‘Animated Time’ did not work for me.
Setting it to Set Range and setting a specific number as Max did work in some cases, but I can’t seem to find the right pattern to reliable calculate the number to use here and usually just start at the full amount of frames for the animation and subtract 1 each time until I find a value that works.