When you try to access SiteGround's FTP server, the following popup window will appear, saying "Unknown certificate" and "Hostname does not match certificate."
You will also see a similar warning message when using other webhosting service providers.
For this issue, SiteGrounds says that you can just ignore it.
I'll be happy to help you with that, that is a normal message due to the way the SSL is setup on our shared servers, it is nothing to worry about so you can just ignore it and continue
So you can just select "Always trust certificate in future sessions." Then this warning message will not appear again.
Just for your reference, if you sign up for a web-hosting plan by clicking on the referral links in this article, I will get a small commission. However, I do not recommend a specific webhosting services just for commission. I’ve been managing WordPress sites hosting by different webhosting service providers including SiteGround and Bluehost for years.
If you want a cost-effective or a cheap but relatively good performance web-hosting, you may want to consider Bluehost’s Shared Hosting. If you sign up for Bluehost's Shared Hosting plan by clicking on this link, you can use Bluehost service with US$1/mo starting from US$2.95/mo.