Just to add to this, if retargeting animations from the UE5 Skeleton it works fine without a crash when you open them, it only happens with the UE4 Skeleton. (atleast for me)
I also tried exporting and then import again, this does not work for all cases 
I see that you are using Synty Characters, check if you have a NEW_Character folder in your pack. They are making alot of Effort to provide them with a UE4 Skeleton, i have 3 packs where they added them in the last weeks.
Makes it a little bit easier to work with out of the box 