I discovered problems with URL rewriting when I use the extension Board Rules.

It's a phpBB bug according to the creator of the extension.

Description of the problem

Default / URL rewriting disabled:

• When I install the extension Board Rules, mysite.com/rules generates 505 Internal Server Error message. mysite.com/app.php/rules works.
• When I first visit mysite.com and click on "Rules" it becomes mysite.com/rules with 505 Internal Server Error.
• If I first click on something else, such as the logo or board index, the path is mysite.com/app.php/rules and works.

URL rewriting enabled:

• The only difference is that "Rules" are now mysite.com/rules no matter what I click on first, but the 500 Internal Server Error persists.

Related topics:
https://www.phpbb.com/customise/db/extension/boardrules/support/topic/141966?p=421571
https://www.phpbb.com/community/viewtopic.php?f=466&t=2307561

The same problem but with 404 error message.
https://www.phpbb.com/customise/db/extension/boardrules/support/topic/132406
https://www.phpbb.com/customise/db/extension/boardrules/support/topic/132996