Hi Osamu!
Thanks for reaching out to us. Let me help you with that issue.
I’m getting a 404 error for the link to your community submission form, so I’m not able to see the problem on my end.
Could you please see if there are any console errors by inspecting the page when seeing that error? We have an article that explains how to do that here > https://theeventscalendar.com/knowledgebase/using-google-chrome-developer-tools/
Additionally, please enable WP_DEBUG and WP_DEBUG_LOG ? This will create a debug.log file inside the /wp-content directory.
After that, try reproducing the issue a few times, so that in case there is an error, it will be saved into the debug.log file. Please share that log file with us so we can see if any problem from there.
Please let me know about that
Thanks,
Victor