Thank you, that did the trick!
A Linux server is unfortunately is not an option, as the client requires the IIS server for other processes.
I’m guessing that it has something to do with WordPress not being able to make a connection to the database, but I don’t know how to solve this.
I’d like to, but the website isn’t due to launch until Monday, so I can’t risk angering my client! Does this screen grab of my dashboard help at all?