Review the port settings. Telnet to the IP address of the unit and login as root:
#> set port ra=*

Any port that is going to have a RealPort device attached, should have:
dev - Needs to be set to rp or prn.
auto - Needs to be set to off

NOTE: You can use the Web Interface to verify the device type as well.

#> set flow ra=*
Any port that is going to have a RealPort device attached, should have all signals off except ixon and ixoff (which can be on or off) and altpin (which should be off for a 10 wire Digi cables and on for anything else).

NOTE: You can use the Web Interface to verify the port setting as well.

#> set auth
Will show settings like the following:

Ind IPaddr          Mask            RealPort  Login  Unrestricted
1   0.0.0.0         0.0.0.0         oooo-ooo -------- oooooooo
                                    o-oooooo -------- oooooooo

Realport should show the lower case letter o for all RealPort ports. The ports are indicated across in groups of eight -- In the example above, all ports are set correctly in the RealPort column except ports 5 and 10 (which are possibly being used for non-RealPort connections as indicated by the hyphen (-) in position 5 and 10).

NOTE: This can only be verified in the command line.

Check for stuck processes.  At the Digi Device/Terminal Server root command prompt type:
#> who