This is because the response of the LoggingHandler added unexpected [ ... ] characters around the values for time, level, logger and message:
This is caused by the way the JSON is created. org.apache.solr.logging.LogWatcher#toSolrDocument creates a SolrDocument which is then formatted with a org.apache.solr.response.JSONResponseWriter.
But the JSONResponseWriter uses the index schema to decide how to format the JSON. We have the following field declaration in schema.xml:
The field type "ignored" has the attribute multiValued set to true. Because of this JSONResponseWriter adds  characters in org.apache.solr.response.JSONWriter#writeSolrDocument
The formatting should be independent from schema.xml