Hi Benjamin,
It looks like your LDAP client is asking the server to return ordered results from looking at this line:
tag=101 err=18 nentries=0 text=serverSort control: No ordering rule
You may want to take a look at the server-side sorting overlay (slapo-sssvlv) and/or the value sorting overlay (slapo-valsort) and see if activating them on the server will fix your problems.