I’ve seen the error several times before. It usually occurs when a plugin or theme has either a PHP error or is being verbose and outputting something it shouldn’t. Did you maybe add some code to your header. If so look to make sure it ok. It almost always some kind of coding error.
Thanks for responding. I’m aware of these potential issues, but AFAIK, nothing has changed in the plugins involved, and certainly nothing in my own plugin. But I “solved” this by issuing an exit-call in my REST handler (which does the output) as opposed to returning a value. So that’ll have to do for now.
(Another option would, of course, have been to simply have a .php file outside of WordPress’ REST framework.)