Do you have any idea what requests are causing this issue?

I've found following code in the source code of ExchangeImpl:

        if ((rCode>=100 && rCode <200) /* informational */
            ||(rCode == 204)           /* no content */
            ||(rCode == 304))          /* not modified */
        {
            if (contentLen != -1) {
                String msg = "sendResponseHeaders: rCode = "+ rCode
                    + ": forcing contentLen = -1";
                logger.log (Level.WARNING, msg);
            }
            contentLen = -1;
            noContentLengthHeader = (rCode != 304);
        }

That would suggest that something is sending 304 and is setting content or passing length of not -1.

Possibly, we could modify https://github.com/tigase/tigase-http-api/blob/a80837c747099682070426756cf8c8eef48c9d1e/src/main/java/tigase/http/java/DummyServletResponse.java#L307 to log stacktrace for calls causing this issue.

Note: I've not observed any entries like that during normal usage of a new version, so I wonder if that is not caused by some external services sending HEAD requests.

Do you have any idea what requests are causing this issue?

It happens on virtually all navigation (/admin, /dashboard, /). Each click/brows one generates at least 3-4 entries (probably number of requests).

Though I just tried and it stopped happening... and it was happening constantly...

I've found cause of this issue (checking if assets/css/js files were modified) and I've applied a fix for this issue.