Error in Digi WR44 Modbus protocol

In Digi TransPort WR44 (v1 and v2) there is an error executing Modbus commands (function code 06) when it is working to serial port.

An example:
Writing to device (Modbus/TCP -Modbus/RTU) - 00 00 00 00 00 06 01 06 00 08 00 AA
Device (Modbus/RTU) answers - 01 06 00 08 00 AA (88 77)
Digi WR44 answers with error (Modbus/TCP ) - 00 00 00 00 00 03 01 86 0B

Function codes 03 and 10 are executing without errors.

Problem exists also in the latest version of firmware.