Should be fixed with https://gitlab.com/bitfireAT/davx5-ose/commit/5709fd38d9df62ca0afc03e764487917fc9cc2e5 / 2.5.1, will send an APK for testing.
For testing:
Install the new APK Open DAVx⁵ and the account Switch to Webcal tab Swipe down to refresh the Webcal list Subscribing to webcal(s):// URLs should now work