If I edit the rule below in Winbox all is good. If I try it in WebFig I get the error message “invalid value in undefined”. Even if I just open the rule and then click “OK” or “Apply” I get the error message. This is via Chrome for the Mac.
10 chain=input action=drop protocol=tcp src-address=!192.168.7.2 dst-address=192.168.7.1 nth=3,1 connection-bytes=200000-0