When I do this
String datum = \"20130419233512\";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern(\"yyyyMMddHHmmss\").withZone(ZoneId.of(\"Europe/B
There's no offset in your source data, and thus OffsetDateTime is not the correct type to use during parsing.
Instead, use a LocalDateTime, since that is the type that most closely resembles the data you have. Then use atZone to assign it a time zone, and if you still need an OffsetDateTime, you can call toOffsetDateTime from there.
String datum = "20130419233512";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyyMMddHHmmss");
LocalDateTime datetime = LocalDateTime.parse(datum, formatter);
ZonedDateTime zoned = datetime.atZone(ZoneId.of("Europe/Berlin"));
OffsetDateTime result = zoned.toOffsetDateTime();