DAVdroid sets its own user agent for requests, which contains the build date. The date is formatted with
SimpleDateFormat("yyyy/MM/dd", Locale.getDefault()). The problem is that in some locales, especially
fa-IR, this format generates strings like
۲۰۱۸/۰۵/۰۵. As a result, the final user agent contains non-ASCII characters which causes IllegalArgumentException in underlying okhttp library.
I suggest using non-localized date format in user agent to fix this problem.