(Just coming here to report a small, low-importance bug since there is no bug tracker.)
For Webcal calendar subscriptions, DAVx⁵ defers to ICSx⁵ to do the actual subscription, but Webcal calendars are also shown with a check mark in DAVx⁵ to indicate the subscription status. However, this does not seem to work if the calendar name contains umlauts, such as this one (“Sun (München)”):
Tapping the check box in DAVx⁵, everything works fine at first, i. e. ICSx⁵ opens and the subscription can be created without any problems. However, after completion, DAVx⁵ still shows an empty check box for the problematic Webcal calendar. The box can be tapped again and ICSx⁵ will happily let you create a duplicate subscription for the very same calendar URL, but the calendar will always remain unchecked in DAVx⁵.