Server ldap/[email protected] not found in Kerberos database

Hi folks,

My question is with regard to an error that I get on an OpenLDAP  
consumer server that uses Kerberos to authenticate to a provider:

   slapd[1668]: GSSAPI Error: Unspecified GSS failure. \
   Minor code may provide more information \
   (Server ldap/[email protected] not found in Kerberos database)

Software versions:
   Operating system: Debian 9 (stretch)
   OpenLDAP slapd: 2.4.44
   Kerberos krb5-user: 1.15
   libsasl2-modules-gssapi-mit: 2.1.27~101-g0780600+dfsg-3

There's a Kerberos key table with keys for an  
ldap/[email protected]  principal that's used for slapd and  
k5start is used to to maintain the Kerberos ticket cache for it. That  
all works fine.

Furthermore, the hostname for the system, srv4, is configured  
correctly, there's almost nothing in /etc/hosts (just '127.0.0.1  
localhost' and a few IPv6 linklocal lines), and the forward and  
reverse DNS entries for this host all refer to srv4.example.com (for  
IPv4 and IPv6).

In /etc/default/slapd I was at first using a statement that said:
   export KRB5_KTNAME=/etc/ldap/krb5-ldap.keytab

But, when I couldn't get rid of the error in question, I changed  
things around a bit and commented out the line or tried this (the  
default):
   export KRB5_KTNAME=/etc/krb5.keytab

Alas, nothing seems to make a difference and slapd insists on  
authenticating itself to the slapd provider as  
ldap/[email protected], which doesn't work.

Could this be a bug? If so, this there a workaround?

Cheers,

Jaap

PS -- If I've come to the wrong place to ask this question, my  
apologies and I would appreciate any hints as to where I should take  
this instead.