asterisk users - May 2010 - call droped if second caller enter meetme conference

Hello Group,
some strange problem i have on my setup. 
If a second caller entering a meetme conference dropping the first one.
my setup is using asterisk 1.6.2.6 and dahdi 2.1.1.1 with realtime, the
conference room numbers storing in a mysql database.
the calls came from a sip provider. there are nothing in logfiles with sip debug
on :(

Has anyone the same problem and a solution for me?

Thanx for all.
Daniel Knoll