Not the parent, but it sounds like they blocked the entire country, including Googlebot's Singaporean IP ranges.
If your server returns different content when Google crawls it compared to when normal users visit, they might suspect that you are trying to game the system. And yes, they do check from multiple locations with non-Googlebot user agents.
I'm not sure if showing an error page also counts as returning different content, but I guess the problem could be exacerbated by any content you include in the error page unless you're careful with the response code. Definitely don't make it too friendly. Whitelist important business partners.
If your server returns different content when Google crawls it compared to when normal users visit, they might suspect that you are trying to game the system. And yes, they do check from multiple locations with non-Googlebot user agents.
I'm not sure if showing an error page also counts as returning different content, but I guess the problem could be exacerbated by any content you include in the error page unless you're careful with the response code. Definitely don't make it too friendly. Whitelist important business partners.