I was able to reproduce it.
I'll apply this patch.
PHP errors
PHP ERROR: Type: 1, File: /etc/inc/util.inc, Line: 2208, Message: Uncaught TypeError: round(): Argument #1 ($num) must be of type int|float, string given in /etc/inc/util.inc:2208
Stack trace:
#0 /etc/inc/util.inc(2208): round('', 2)
#1 /usr/local/pfSense/include/www/bandwidth_by_ip.inc(172): format_number('', 2)
#2 /usr/local/www/bandwidth_by_ip.php(36): printBandwidth('wan', 'local', 'in', '', 'iftop')
#3 {main}