Disconnects with timeout if connecting from outside

No config changed, but suddenly I noticed that if I connect to my jabber server from outside (external IP, port-forwarded to jabber server) it will disconnect soon with timeout error. Sometimes it shows roster, sometimes I even could download vCard, but still it disconnects sooner or later. Should I connect from the inside (192.168 network) it all works without a problem.
{loglevel, 2} shows no error in logs.
{loglevel, 5} shows a lot of messages, but I think that these lines have something to do with this problem, even though it does not happen if I connect from inside (192.168 network)

=INFO REPORT==== 2014-07-30 18:25:41 ===
D(<0.371.0>:ejabberd_receiver:320) : Received XML on stream = " \t "

=INFO REPORT==== 2014-07-30 18:25:41 ===
D(<0.371.0>:shaper:61) : State: {maxrate,1000,998.9598907386801,
1406730282789325}, Size=3
M=2.9968829143257185, I=58925.328

any help greatly appreciated

Additional info:
ejabberd (2.1.10-4+deb7u1)
auth through LDAP

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