When trying to delete post permanently, "Insecure redirect" error occurs if redirect link contains #p anchor.

That is because of https://github.com/phpbb/phpbb/blob/c9db94f91cd4e682c8c2fc2d768eab52042d5ec7/phpBB/includes/mcp/mcp_main.php#L1248 condition which has an issue with parentheses that causes calling function substr() with a bool 3rd argument: $redirect = substr($redirect, 0, $anchor_pos);

After this, $redirect link becomes a dot from for example ./viewtopic.php?t=1&p=3#p3 link.