ldapsearch -D "cn=directory manger" -W -b cn=config objectClass=nsDS5Replica
nsDS5ReplicaPurgeDelay is not set listed in the output :(. It must be at the default value of one week?
Also, you mentioned that the agreement might have been disabled. What field of the nsds5replicationagreement class shows that?
Given the error in the log, and the low likelihood of the agreement being disabled for a week, what else can cause a node not to find a CSN?