It's not about the .online TLD being "weird". The problem is that it was free. That's going to attract a swarm of fraudsters, spammers, etc, and then turn into a strong "this is probably fraud" signal in all kinds of fraud scoring systems.
There are lots of domains out there other than .com that are just fine.
Probably this is what's happened here. Either the OP's domain was previously used for shady activities, or the almost-free stigma puts the whole .TLD in the grey list of high-risk assets. Probably is also explains the nuclear behavior of the registrar (suspension).
Free is good, but sometimes it's not.
.online, .top, .xyz. info and .shop are some of the top TLDs that scammers use, precisely because of their rock bottom registrar fees that make them attractive for sites that have a shelf life of a few hours or a few days before being blocked. As a result, many places have a blanket "suspicious" flag for fresh domains under these TLDs.
If you plan on building a legit site, do not use any of these cheap TLDs.