Ok... it really could be something PHP 7.3 specific and it is definitely related to UTF-8 chars. I suppose, the error may appear at the stage when miniBB tries to split a string. At some of my older development stages I've met and worked out this issue, but now it seems to come back.
As it's announced
PHP 7.3 release, there were introduced improvements for
PCRE extension,
MB-string functions and also there are
several deprecated features for string-parsing functions. All of these updated could be the reason of this issue.
Alas, I couldn't test UTF-8 texts on PHP 7.3 now, but I will try to do it in the nearest future.
You could try one thing on your own, if you have enough experience... In bb_func_txt.php, miniBB uses this code to split the string by chars, this also includes UTF8-chars:
$str=preg_split($splitExpression, $fragment, null, PREG_SPLIT_NO_EMPTY);
$splitExpression equals to '//u' by default, but this could be re-defined in setup_options.php. I'm not sure what has to be work-out for PHP 7.3, but probably the error comes from this place. When the script tries to split a string, for some reason it couldn't do it and gets an empty value. That's why in result an empty message appears.