Each time I run maintenance/refreshLinks.php, I get this error a few times. (I removed some of the path for privacy reasons.)
Warning: A non-numeric value encountered in /includes/parser/CoreParserFunctions.php on line 901
Line 901 in 1.29, part of CoreParserFunctions::pad():
$length = min( $length, 500 ) - mb_strlen( $string );
It occurs at between lines 4700 and 4800 and between lines 12300 and 12400.
I did some research on this error. Apparently, it's related to php7.1. I found this discussion on Github - https://github.com/dompdf/dompdf/issues/1272 that seems to indicate the use of arbitrary string values was previously silently cast to the number 0 if used in a mathematical expression, but as of PHP 7.1 is considered an error. In Dompdf this happened with the css sting "auto". Perhaps the use of "null" instead of 0 happens here.