couchdb replication create many "doc_write_failures"

because of a server change I try to bring my couchdb to an other server. To bring document to the new server I like to use the replication feature.

source and target are a couchdb 2.1.1

The sourche-DB has 5275 documents, on the target-DB there are after replication only 2042 documents.

I tried to update a DB and let replication create a new one.

In the replication document I can see many doc_write_failures, what can cause this? Are there any other log where I can see what goes wrong?

{
  "_id": "sync 1.0",
  "_rev": "2-38c27890daa785c49925a1272d745e10",
  "user_ctx": {
    "name": null,
    "roles": [
      "_admin",
      "_reader",
      "_writer"
    ]
  },
  "source": {
    "headers": {
      "Authorization": "Basic YWRtaW46TURTdGF0"
    },
    "url": "XXX"
  },
  "target": {
    "headers": {},
    "url": "XXX"
  },
  "create_target": true,
  "continuous": false,
  "owner": null,
  "_replication_state": "completed",
  "_replication_state_time": "2019-10-15T12:28:54Z",
  "_replication_stats": {
    "revisions_checked": 5848,
    "missing_revisions_found": 5848,
    "docs_read": 5848,
    "docs_written": 2324,
    "changes_pending": null,
    "doc_write_failures": 3524,
    "checkpointed_source_seq": "24287-g1AAAABSeJzLYWBgYMxgTmEQTM4vTc5ISXLIyU9OzMnILy7JAUoxJTLksTD8B4KsDOYkBoa4-7lAUXZjg1QgMMKmKwsA914aPQ",
    "start_time": "2019-10-15T12:28:30Z"
  }
}