403 Forbidden when trying to send logs using Filebeat

I'm using Filebeat OSS licenced version 7.10, and AWS ElasticSearch 7.10.
I use a reverse logging proxy and set up auth like this:proxy_set_header Authorization "";
When configuring Filebeat, I use Basic Auth (username and password, base 64 encoded) and use that to send logs to ElasticSearch.
I get a 403 error with the following message:

Authorization header requires 'Credential' parameter. Authorization header requires 'Signature' parameter. Authorization header requires 'SignedHeaders' parameter

What could be the reason for this error?
Thanks