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
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