I got this error as well for 2.6.3 – after I propagated a new URL through the internet. You have to have the EXACT match of your website name in the command bar as is on the WordPress settings – General – WordPress Blog Address (URL) field – and it is case sensitive.
I had https://www.TheCreativePenn.com in the field and actually it should have been https://www.thecreativepenn.com – when I changed it, everything works fine again.
I hope that helps some people – I tried a whole load of other things first that were complicated – but it was this simple! so try this first and then try the re-installing etc etc.
