[development] blank screen at line 1

while developing a module, I have just started to recieve white screen every time I submit a form (besides form gets submited). In some cases I have been able to see this error: warning: Cannot modify header information - headers already sent by (output started at /var/www/ccc/sites/all/modules/comptacau/comptacau.module:1) in /var/www/ccc/includes/common.inc on line 141. but I file comptacau.module have no white spaces at start. Any hint of how to track this problem? thanks PS: memory limit is set to 256 just in case it was a memory problem, but apache reports no problem with memory. -- *Envellim quan els records superen les esperances. *Als llocs desconeguts només s'hi arriba per camins desconeguts.