Prepare join failed. Got HTTP status 500 from REST

1. Create a 2 node cluster -
rebalance params :

{'knownNodes': 'ns_1@172.23.97.52,ns_1@172.23.97.53', 'ejectedNodes': '', 'user': '', 'password': ''}

2. Create a bucket
3. Disable Auto Failover -
timeout=120&enabled=false&failoverOnDataDiskIssues%5Benabled%5D=false&maxCount=1&failoverServerGroup=false
4. Turn on n2n and control on .52
/opt/couchbase/bin/couchbase-cli node-to-node-encryption -c http://localhost -u Administrator -p --enable
/opt/couchbase/bin/couchbase-cli setting-security -c http://localhost -u Administrator -p xx --set --cluster-encryption-level control
5. Add documents to default bucket
6. Add .56 to the cluster
adding remote node @172.23.97.56:18091 to this cluster @172.23.97.52:8091
[2022-01-17 21:06:00,856] - [rest_client:1882] INFO - rebalance progress took 10.02 seconds
[2022-01-17 21:06:00,857] - [rest_client:1883] INFO - sleep for 10 seconds after rebalance...
[2022-01-17 21:06:10,948] - [rest_client:1055] ERROR - POST http://172.23.97.52:8091/controller/addNode body: hostname=https%3A%2F%2F172.23.97.56%3A18091&user=Administrator&password=xxx&services=kv headers:

{'Content-Type': 'application/x-www-form-urlencoded', 'Authorization': 'Basic QWRtaW5pc3RyYXRvcjpwYXNzd29yZA==', 'Accept': '*/*'}

error: 400 reason: unknown b'["Prepare join failed. Got HTTP status 500 from REST call post to https://172.23.97.56:18091/engageCluster2. Body was: \\"\\\\\\"Unexpected server error, request logged.\\\\\\"
""]' auth: Administrator:xxxx

This test passed on build 6.6.5-10068 and fails on 6.6.5-10076

Change log between 2 builds:
http://changelog.build.couchbase.com/?product=couchbase-server&fromVersion=6.6.5&fromBuild=10068&toVersion=6.6.5&toBuild=10076&f_asterixdb=off&f_build=off&f_cbas=off&f_cbas-core=off&f_eventing=off&f_eventing-ee=off&f_go-couchbase=off&f_indexing=off&f_ns_server=on&f_query=off&f_testrunner=off