2.3.35 -> 2.4.7 replication: null_callback : error code 0x10

Testing interoperability between 2.3.35 and 2.4.7.

Consumer is 2.4.7, provider is 2.3.35.

I get messages in the 2.4.7 log that look like this:

Jan 15 13:07:47 mippet slapd[13134]: null_callback : error code 0x10 Jan 15 13:07:47 mippet slapd[13134]: syncrepl_updateCookie: rid=002 be_modify failed (16) Jan 15 13:07:47 mippet slapd[13134]: do_syncrepl: rid=002 retrying (4 retries left) Jan 15 13:07:52 mippet slapd[13134]: null_callback : error code 0x10 Jan 15 13:07:52 mippet slapd[13134]: syncrepl_updateCookie: rid=002 be_modify failed (16)

Seems to be related to adding/changing/deleting on the provider, although the changes do propagate. Is this fixed by 2.3.40, or is it a separate issue in 2.4.7?