The Apache/PHP error log is below.
Not sure if it helps. I do not see any PHP code issue in fancy_stop_spam.inc.php.
Corrected later: Line 957 of the file uses the short array syntax to declare $patterns. This syntax requires PHP 5.4+. Editing the code block to use $patterns = array(...) instead of $patterns = [...] fixed my issue.
// Extension was previously installed. It caused blank page. So config.php was edited to disable extension hooks. Then extension was disabled on forum admin page.
// Today at 14:47, extension was enabled on forum admin page by clicking on enable link, which caused blank page to appear. Then I just typed URL of different forum pages in browser (forum main page, etc.), which all appeared as blank pages.
[Wed May 04 14:47:08 2022] [error] [client *] PHP Parse
error: syntax error, unexpected '[' in
*/forum/extensions/fancy_stop_spam/fancy_stop_spam.inc.php
on line 923, referer:
*/forum/admin/extensions.php?section=manage
[Wed May 04 14:47:18 2022] [error] [client *] PHP Parse
error: syntax error, unexpected '[' in
*/forum/extensions/fancy_stop_spam/fancy_stop_spam.inc.php
on line 923
[Wed May 04 14:47:31 2022] [error] [client *] PHP Parse
error: syntax error, unexpected '[' in
*/forum/extensions/fancy_stop_spam/fancy_stop_spam.inc.php
on line 923