Thanks @RGJ and @simon for the advice. I sent this post to Siteground Support staff and they finally figured out what caused it:
I checked all of the suggestions and noticed that there was mentioned another client of ours who has encountered the same issue. In his case, the problem was resolved by updating the IP which the domain should resolve to in the local DNS zone on our server.
After adding the following DNS record:
discourse.saladebriefing.com.br. 14400 IN A 45.32.168.108
The connection was established successfully and immediately by your plugin.
So, there you go. Special thanks to Stela C. from Siteground support who had the patience to carefully review everything and finally figure it out.