with a little grain of more information someone might have the ability to check - but with the given information, it looks like a local problem. Because I do not see that on my 4.0.2
Sorry. This a production system so I just put it back on 4.0.1 to keep it working. I just briefly went back to 4.0.2 to reproduce, which it did instantly. I wiped all the data first and immediate get this in the log repeatedly.
2021-02-23 14:20:19,368 WARN : org.graylog.storage.elasticsearch6.MessagesAdapterES6 - Failed to index message: index=<graylog_0> id= error=<{“type”:“mapper_parsing_exception”,“reason”:“failed to parse field [timestamp] of type [date]”,“caused_by”:{“type”:“illegal_argument_exception”,“reason”:"Invalid format: “2021-02-23 19:18:28.726"”}}>