Hello Andrey and sgt_pfk,
Thank you so much for your quick answer. SGT, based on what you advised, I checked for every possible space but didn't find any. But your assumption is not totally wrong, in fact the problem (I think) might come from the code not being "fully compliant" with my version of PHP (the latest though). I had php error turned on (E_ALL and E_NOTICE), E_NOTICE returned an error as I wasn't uploading a picture => Notice: No file uploaded in Unknown on line 0
(and not BlogsNotice... sorry, my mistake Andrey:)
This error placed at the top as html, most likely caused the second error:
Warning: Cannot modify header information - headers
already sent in /home/www/website/inc/design.inc.php on
line 487
Turning off error logging in php.ini did the trick.
Again, thanks to both of you, I really appreciated your help!