如何使用htaccess重写规则修复“服务器找不到请求的页面”错误

Intermittently I get the error 'The server can not find the requested page' when trying to view pages on my website. Generally pressing refresh resolves the issue.

I suspect this is caused by some of the rewrite rules in the htaccess file, but I can't see what is wrong. The contents of the file is below:

Options -Indexes

RewriteEngine on
RewriteBase /
RewriteRule ^sequences$ sequences.php [L]
RewriteRule ^settings$ user-settings.php [L]
RewriteRule ^variables$ user-variables.php [L]
RewriteRule ^help/([^/\.]+)/?$ help/help.php?content=$1 [L]
RewriteRule ^help$ help/help.php [L]
RewriteRule ^help/$ help/help.php [L]
RewriteRule ^register$ register.php [L]
RewriteRule ^login$ login.php [L]
RewriteRule ^terms$ terms.php [L]
RewriteRule ^trigger-webhook/([^/\.]+)/([^/\.]+)/([^/\.]+)/?$ trigger-webhook.php?k=$1&s=$2&t=$3 [L]

An example of a page which has returned the error is /help/conditions, but I have also experienced this from other pages. I'll come back with more examples as they happen.

Does anyone know what might be causing this issue? Is the contents of the htaccess file correct?