your logo URL is HTTP – it needs to be updated to HTTPS for the browser to stop blocking it.
Upload a new copy of your logo with a different filename to replace your existing logo.
Siteground has a plugin called SG Optimizer and one of the plugin’s settings under Environment Optimization is to “Fix Insecure Content”. Enabling this setting seemed to have fixed the issue. It was definitely due to the lack of https. Thanks for your help!