This error occurs if there is any out put to the page before a header() or session_start() function.
The error reports:
output started at C:\xampp\htdocs\lentz\admin\articles_Insert.php:26
So I would start looking around line 35 and line 26, Look for and echo() command, or blank space between code blocks like:
28- <more PHP code>...
that blank line at 26 above will cause a similar error. if you can spot the problem, compress you page in a zip archive and attach it to your replay and I will take a look.