When a user includes the U+FFFE character or U+FFFF character in a post, forum search displays a bunch of errors at the top of the page when this post is in the search results. 

The first error is:

[phpBB Debug] PHP Warning: in file [ROOT]/vendor/s9e/text-formatter/src/Utils.php on line 161: DOMDocument::loadXML(): Char 0xFFFE out of allowed range in Entity, line: ...

Apparently users can enter these characters, but forum search cannot properly handle this.

It seems like an unsolved problem that has been around for a long time. See also my forum post. Doing a google search on "Char 0xFFFE out of allowed range in Entity" (include quotes) finds more forums with this issue.