Many replication related bugs have been corrected since 2.4.11, which is a rather old release now. This bug may well be fixed in the latest version 2.4.18.
I recommend you try this latest release. I think you will find debian packages available either on backports or from third parties.
Ok thanks you for your feedback. I have now readded the whole ldap tree with slapadd, and it hangs again I will upgrade to 2.4.18
EK