After adding a new blog post today, I tried to backup my website (robertfairhead.com) and received an error message:
500 Application Error Exception: Invalid security token
The last successful backup was on 17 January 2022 (after my last blog post).
I tried clicking the "Reset temporary storage" option on the error screen, but that didn't work.
Before I added today's post, I had updated WordPress, my Divi Extra theme and a number of plug-ins. I wondered if this could have caused a problem so I tried cleaning the cache, but without success.
I then tried logging out of WordPress and back in again. And, thankfully, this worked.
I'm posting this as a public ticket in case it's helpful for someone else with a similar problem.