hi mlohbihler,
i am using BACnet4j too. very thanks to you.
but, i have similar problem, Mr.sebastian's

i received who-has packet.

0x81, 0x0b, 0x00, 0x19, 0x01, 0x20, 0xff, 0xff,
0x00, 0xff, 0x10, 0x07, 0x09, 0x00, 0x1b, 0x3f,
0xff, 0xff, 0x3d, 0x05, 0x00, 0x45, 0x4c, 0x45,
0x43
It refers to the information below.
device instance low limit : 0
device instance high limit : 4194303
object name : ELEC

but BACnet4j occurred exception.

Exception in thread "main" com.serotonin.bacnet4j.exception.BACnetException: Error while creating APDU: 
	at com.serotonin.bacnet4j.npdu.ip.IpMessageControl$IncomingMessageExecutor.runImpl(IpMessageControl.java:500)
	at com.serotonin.bacnet4j.npdu.ip.IpMessageControl$IncomingMessageExecutor.access$0(IpMessageControl.java:464)
	at com.serotonin.bacnet4j.npdu.ip.IpMessageControl.main(IpMessageControl.java:770)
Caused by: com.serotonin.bacnet4j.exception.BACnetErrorException
	at com.serotonin.bacnet4j.type.Encodable.popStart(Encodable.java:109)
	at com.serotonin.bacnet4j.type.Encodable.readWrapped(Encodable.java:323)
	at com.serotonin.bacnet4j.type.Encodable.read(Encodable.java:185)
	at com.serotonin.bacnet4j.type.Encodable.readOptional(Encodable.java:210)
	at com.serotonin.bacnet4j.service.unconfirmed.WhoHasRequest.<init>(WhoHasRequest.java:109)
	at com.serotonin.bacnet4j.service.unconfirmed.UnconfirmedRequestService.createUnconfirmedRequestService(UnconfirmedRequestService.java:50)
	at com.serotonin.bacnet4j.apdu.UnconfirmedRequest.<init>(UnconfirmedRequest.java:62)
	at com.serotonin.bacnet4j.apdu.APDU.createAPDU(APDU.java:38)
	at com.serotonin.bacnet4j.npdu.ip.IpMessageControl$IncomingMessageExecutor.runImpl(IpMessageControl.java:497)
	... 2 more

i think BACnet4j can't find context start/end tags.
this problem occurs only 'who-has packet with device instance low/high limit'.