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It's not coming from our software, I can tell you that for sure. This comes from a third party plugin, most likely a system plugin written by someone who is oblivious to the fact that Joomla! is expected to create output other than plain HTML, as I had detailed two and a half years ago in https://www.dionysopoulos.me/common-mistakes-writing-joomla-plugins.html (see “There is more to Joomla than HTML output” in that article). As I say in there, Joomla! has really been to generate non-HTML output ever since Joomla! 1.0 back in 2005 – someone didn't get the memo for no less than 19 years!

Unfortunately, troubleshooting this is rather obnoxious. You need to start disabling one system plugin at a time and test the API URL. If the problem persists, re-enable the plugin you just disabled and move on to the next one. Once you find the plugin that causes the problem you need to contact its developer to fix it. Link them to my article (see link above) so they have a clear reference of what to do and what not to do in their code.

You're welcome! I am glad I could help :)