Ok, so I've done a fresh install of 2.3.1 and installed the TinyMCE package on its own and the Fatal Error comes up when inserting a link, so it looks like there's something not set right in the package for TinyMCE
No other errors in the log and the other files I mentioned are not there, so those must have been left over from a prior version
Investigating the popup the error displays inside of, that's loaded from /assets/components/tinymce/jscripts/tiny_mce/plugins/modxlink/link.php?v=231pl
The error appears at the end of the page that's loaded, but to check I dropped some simple tracers around the output, so that the file read
echo 'a'; //tracing
echo $o;
echo 'b'; //tracing
die();
And the output reads
</html>b<br />
<b>Fatal error</b>: Undefined class constant 'LOG_LEVEL_ERROR' in <b>/usr/www/users/sfxscm/new/core/model/modx/error/moderrorhandler.class.php</b> on line <b>111</b><br />
So it seems like it's occurring as a result of the 'die()'