I just spent the last 30 minutes going thru this. I was able to recreate the error you showed. The error only occurred when trying to summarize the IPs in Group Policy using "Custom network firewall & shaping rules". I am unable to combine multiple IPs into the one rule and the error message you showed is generated. This happens regardless of firmware version.
When placed in Security Appliance > Configure > Firewall, the rule works without any issue. This leaves me to believe that it is a problem with group policy looking for only ONE IP when creating custom rules. Have you tried placing the rule in the Layer 3 section of the Security Appliance > Configure > Firewall page? This will apply to ALL groups on your MX unless you create a custom rule under group policy. If everyone is to be blocked from these IPs, my recommendation would be to place it there.
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