It’s a really weird issue.
To troubleshoot it, I need to make changes in the theme files but I couldn’t do it due to the error message as shown in the attached screenshot.
Could you please share me your site FTP details so that I can make the changes in the theme files?
I troubleshoot it and found out that this is happening because of aggressive caching by SG Optimizer used on your site but I can’t clear or purge its cache as displayed in the attached screenshot.
So please contact the plugin developer or SiteGround Performance services to resolve it.
Then it seems another type of cache exist on your site causing this issue.
You can visit the below CSS file
Then see the same CSS file on your server using FTP that the file on the server is updated but above CSS file is old one due to cache.