I confirmed the message below (This post is via “anonymoX”).
Forbidden
You don’t have permission to access this resource.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.
新しいアカウントを作ってみてはどうでしょうか？
I cannot browse this forum as an unregistered user also. And it is difficult to post it in Japanese.
When I post in Japanese, it will be displayed as below.
“The requested page could not be found.”
Someone may be interfering. 
Hi Ernie,
I don’t know what the cause of the issue might be but browsing the forum as an unregistered user works on my PC.
As you might know, some of the content is hidden for unregistered users but I can access all the topics without problems, even without an account.
Just to check if your web browser is ok, you could try to browse the site in Incognito mode:
Chrome:https://support.google.com/chrome/answer/95464?co=GENIE.Platform%3DDesktop&hl=ja#:~:text=You%20can%20also%20use%20a,Press%20⌘%20%2B%20Shift%20%2B%20n.
Firefox: https://support.mozilla.org/ja/kb/private-browsing-use-firefox-without-history
I also stumbled upon this extension:
https://chrome.google.com/webstore/detail/anonymox/icpklikeghomkemdellmmkoifgfbakio
https://addons.mozilla.org/en-US/firefox/addon/anonymox/?src=homepagedl
If you have it installed you might want to try to remove it to check if it’s the source of the issue.
If the forum works fine in incognito, you might want to try to delete your web cache/temporary browser files.
I am always very grateful for your contributions, I hope the issue will be resolved soon.
Thank you artins90-san.
I tried those methods but it still didn’t work.
Can you give it a try now Ernie? Can you give the steps you use to reproduce the error?
When I press the “Preview” or “Submit” button, the following display appears.
The “Forbidden” problem has been resolved but…
Add 後半戦 and then I press the “Preview” or “Submit” button, the following display appears.
後半戦
後半戦Seems to work?
When you see that ‘Requested Page Cannot Be Found’, what is the URL in the address bar?
Fixed, looks like mod_security really didn’t like that…