[HELP]: IAX Packet decrypt failed

Two Asterisk Servers each of them with one Digium TDM400 card are connectd via IAX over the internet via ADSL (upload 600kB/s/ download 3500kB/s.

If a user wants to call, he dials the local telephone number connected on one Asterisk server (TDM400). Asterisk bridges the call immediate over the internet (IAX) to the second Asterisk server which then bridges the call to the TDM400 card, which dials then a regular telphone number on the other side. If the receiver pick-up the phone the call is established.

Everything works fine for the first 5-10 second of an established call. After that following error messages on both Asterisk server appear:

[color=red]chan_iax2.c:6515 socket_read Packet Decrypt failed[/color]

Of course the line will hang up after another 10 second.

From my point of view one IAX call uses arround 50kB bandwith. So it can’t be a bandwith problem.

Has anybody made the same expierence? How can this be solved?

PS: using Asterisk-1.2.9, Zaptel-1.2-21