Invalid Etag format prevents its validation and correct usage.

[pablolagos]

When the Etag for static files is generated it is sent without quotes, so later the validation is not done correctly:

Response headers:

HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 104006
Content-Type: text/css; charset=utf-8
Etag: 8JmZhmJvb3RzdHJhcC5taW4uY3NzTW9uLCAyNyBBcHIgMjAyMCAxNTowMDowNiBHTVQ=
Last-Modified: Mon, 27 Apr 2020 15:00:06 GMT

As described in RFC 7232, the Etag must be in quotes:

https://tools.ietf.org/html/rfc7232#section-2.3