Is it an RP mount, or your regular mount? What is the name of the mount? We need more details as Dave mentioned.
Have you tried a clean reinstall? Delete folder > Restart PC > Reinstall If you are still crashing, are you using IMG or WZ files format
I've replicated the issue. It seems that there is an issue with the Schwarzer cape and sitting animation. Moving this over to Report Bug. To avoid crashing in the meantime, you can unequip the cape.