connection error by more than 4 nodes

Hello,

i have 1 tinc-node which is reachable from internet.
If more than 3 clients connect to the "master", the fourth client (laptop) could not connect the master and
so on the others.

This is the debug-message from the client:
Trying to connect to master (x.x.x.x port xxx)
Connected to master (x.x.x.x port xxx)
Sending ID to master (x.x.x.x port xxx): 0 laptop 17
Sending 12 bytes of metadata to master (x.x.x.x port xxx)
Got ID from master (x.x.x.x port xxx): 0 master 17
Sending METAKEY to master (x.x.x.x port xxx): 1 94 64 0 0
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Sending 781 bytes of metadata to master (x.x.x.x port xxx)
Flushing 793 bytes to master (x.x.x.x port xxx)
Connection closed by master (x.x.x.x port xxx)
Closing connection with master (x.x.x.x port xxx)

log from master:
1267735212 tinc.xxx[8330]: Connection from x.x.x.x port 56601
1267735212 tinc.xxx[8330]: Sending ID to <unknown> (x.x.x.x port 56601): 0 master 17
1267735212 tinc.xxx[8330]: Sending 12 bytes of metadata to <unknown> (x.x.x.x port 56601)
1267735212 tinc.xxx[8330]: Flushing 12 bytes to <unknown> (x.x.x.x port 56601)
1267735212 tinc.xxx[8330]: Got ID from <unknown> (x.x.x.x port 56601): 0 laptop 17
1267735212 tinc.xxx[8330]: Sending METAKEY to laptop (x.x.x.x port 56601): 1 94 64 0 0 xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
1267735212 tinc.xxx[8330]: Sending 781 bytes of metadata to laptop (x.x.x.x port 56601)
1267735212 tinc.xxx[8330]: Got METAKEY from laptop (x.x.x.x port 56601): 1 94 64 0 0 xxxxxxxxxxxxxxxxxxxxxxxxxx
1267735212 tinc.xxx[8330]: Error during decryption of meta key for laptop (x.x.x.x port 56601)
1267735212 tinc.xxx[8330]: Error while processing METAKEY from laptop (x.x.x.x port 56601)