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So from what I can see, Avast simply mentioned in that forum that they have blocked our IP addresses due to suspicious behavior (they recognize our IP structure as spammy, even though its not
No, avast has blocked the IP because there are malicious on it, not just because of a suspicion.
I would say they're definitely blocking this on their end due to this misunderstanding of our IP structure
Sure, avast is blocking that IP but not because avast doesn't understand their structure.
Basically, just tell them its not malicious and has been verified as ok from almost all of the malware checkers they provided in the links in that forum you sent
The links I have posted show very clearly that things are not ok.

It is very clear that Squarespace is not honest about things and/or they don’t have a clue about things.

I run checks on my websites/IP’s and the ones I maintain for others on a regular base.
About two years ago I saw malicious things where happening on the same IP.
I contacted my host, and within 24 hours they gave me a new IP, monitored the other websites and removed the malicious ones.

And my question now. Sending a ticket will work? Will this ip going to be unblocked?
Yes, avast will have a look at your website and if nothing malicious is found they will allow it. They will not unblock the IP as long as there is malicious going on there.