This may be due to the U10 default buffer size of 66 bytes being to small for the BACnet over LON requirement of 208 byte buffers. You can change the U10 buffer sizes in the LonWorks Interfaces Applet in the Windows Control Panel. Since the U10 has limited RAM making the buffer sizes larger may affect performance.
The U60 DIN (FT-10 channel) can be used instead. The U60 has a lot more RAM and defaults to 255 byte buffers.