You may experience this or similar error message on Your PHPBB forum or in your error_log file.

Deprecated: Assigning the return value of new by reference is deprecated in /home/*****/public_html/includes/captcha/plugins/phpbb_captcha_gd_plugin.php on line 55
What helped me is to go into mentioned path: /public_html/includes/captcha/plugins/

and there should be like 6 files. Go thru all files and Find + replace "=&" by "=" in these files. First make backup of original files.

Then go to PHPBB admin control panel and clear cache (button on admin area homepage -

Does it helped you?