By setting "error reporting" to maximum, i found indeed what was the problem.
The error message was:
Fatal error: Maximum execution time of 30 seconds exceeded in ..\com_joomdoc\classes\DOCMAN_html.class.php on line 199
By analyzing the php code on line 199, I have found that the loading of images option list was exceeding 30s.
I checked the /images/stories directory for image file size. By removing some of the images whose size was too big, I resolved the problem.
The page still takes some time to load but I think it may be caused by deprecated function eregi()
Deprecated: Function eregi() is deprecated in ..\administrator\components\com_joomdoc\classes\DOCMAN_html.class.php on line 199
Thank you for you help makl