I understand that in the first place you can think about the vulnerability of the server, but besides bitcoin, there was still a lot of things that could be stolen, but nothing was stolen, moreover, not even all of the bitcoin was stolen, so this is anything but not a server vulnerability. Perhaps the problem is indeed in weak entropy, but what exactly in my case, I don’t understand.
Fair point, but I was eliminating various possibilities before coming to this conclusion. If you're talking about weak entropy, the only possible point of failure lies with BitPay, Bitcoin Core does have a pretty robust system when it comes to RNG by taking randomness from multiple sources. It would be hard to imagine that it would be a point of failure and besides the there are no address reuse (I presume) and it would eliminate the possibility signature nonce re-use (nonces are deterministic anyways).
Taking in mind that the possibility of Bitcoin Core being flawed is fairly low, the only two possible factors would be any vulnerabilities within your server or BitPay. Regardless, eliminating possibility based on the possible psychological thinking of the attacker wouldn't be effective in determining the actual cause. Perhaps they are only interested in those, perhaps they didn't want to draw attention to themselves, etc.