yep. for the same reasons you can’t open a 24 file in 2020. in 21 they rewrote the way saving files works, and they are now half the weight (so twice the compression). onscreen it looks the same, but in the background, not exactly 
you can try this
the first link is an extension for SU24 to export to SU20
the second link is a standalone tool that will convert the 24 file to 20