Further observations show that...

1. processing add requests seems to behave like expected.

2. it depends on the assertion filter sent in the control value:

$ ldapmodify -H ldap://localhost -D "uid=Mstroeder,dc=stroeder,dc=de" -W -f uidnumber_clash.ldif -e 'assert=(cn=*)' modifying entry "uid=ttester,dc=stroeder,dc=de" ldap_modify: Operations error (1) additional info: unique_search failed

$ ldapmodify -H ldap://localhost -D "uid=Mstroeder,dc=stroeder,dc=de" -W -f uidnumber_clash.ldif -e 'assert=(objectClass=*)' modifying entry "uid=ttester,dc=stroeder,dc=de" ldap_modify: Constraint violation (19) additional info: some attributes not unique