Null pointer exception when trying to perform the remote reindex

I am facing nullpointer exception with ‘null’ reason when trying to perform cross-domain remote-reindex. both the domains are on ES 7.8

Any idea how I can make it work? Thanks in advance
{
“error” : {
“root_cause” : [
{
“type” : “null_pointer_exception”,
“reason” : null
}
],
“type” : “null_pointer_exception”,
“reason” : null
},
“status” : 500
}

I have the same issue with a 6.8 local cluster and a 6.5 remote cluster on AWS Elasticsearch service.

Do you find any solution/ workaround from your side ?