There’s more than One Reason For this i will provide you with two solutions (Meta-link Note):

Solution One : 

1. Backup and edit the forms configuration file (Default= formsweb.cfg)
2. Modify the HTMLdelimiter to have only one character as a value 

HTMLdelimiter=%

3. Save and exit the file.

Solution Two :

Edit oracle_apache.conf and comment out the include related to the forms.conf  as the following :

# Oracle Forms Developer
#include “/oas/product/oas2/forms/server/forms.conf”

Thank You
osama mustafa