This is about a forum that's a phpbb, whatever that means. I have not touched the coding or tech of this forum in several years. This just started happening on its own earlier today. Please help me. What do I do?

Here is the error message in this pastebin. Also, when anyone tries to post or send a private message, it always gives you an error message that there are too few characters in your message.

https://pastebin.com/dfwYxBNV

If this means anything, when I go into my Administration Control Panel, it shows these error messages:

Your phpBB installation is not up to date.

HTTP input character conversion is improperly configured

mbstring.http_input must be set to pass. You can check the current value on the PHP information page.

HTTP output character conversion is improperly configured

mbstring.http_output must be set to pass. You can check the current value on the PHP information page.