From experience, I no longer allow CCleaner to automatically delete issues it discovers while performing Registry Cleaner task.
The other day I was curious to see what would come up. I noticed some invalid firewall entries. Not long ago, I reset my Windows firewall to default so I'm a bit puzzled why there would be any invalid entries given firewall entries (should've) been removed after the reset.
Is it ok to delete the invalid firewall rules?
the firewall-rules probably are being flagged because of missing files. that is the only time that i have seen any firewall-rules flagged, when a particular file that the rule applied to was missing. so yes, it would be OK to remove the the invalid firewall-rules.
you could run the "FRST" program. it also will show "invalid" firewall-rules, rules that have missing files and therefore are invalid (in the "additional" scan-log):
https://www.bleepingcomputer.com/download/farbar-recovery-scan-tool/
using ccleaner's registry-cleaner has never caused me any problems.
Thanks!