I think pjax has something to do with content, fetched via ajax and then included to the DOM. Anyway. A header information need do be sent before some output happens, so it´s invalid at this point and will always throw warnings. I think I'll leave it commented out and will report if there are any problems, till the next update...I moved the header() function from line 80 to line 70 (before any html output in this file) and it seems to fix the problem.