After testing phpBB on a virtual partition on my computer, I uploaded the site to my server. Both sites ran CentOS 7. I found phpBB failed any time I went to the ACP: nothing would appear in the browser. I am running CentOS 7.8.2003 on both systems, but the server is hosted under CPanel.
After much head-scratching, it turned out that the CPanel install of PHP 7.4 did not include the iconv PHP module. This caused this error to appear:
PHP Notice: iconv(): Wrong charset, conversion from `utf-8' to `utf-16be' is not allowed in /home/bluecoat/public_html/vendor/patchwork/utf8/src/Patchwork/PHP/Shim/Iconv.php on line 175:
Adding the iconv extension fixed the problem. Y'all might want to list iconv as a requirement in the documentation.