For some unknown reason, I've come back to the issue a week later and it's fixed itself! I honestly have no idea why, but I would be sharing a screenshot of the dialogue box otherwise. I haven't made any changes to the site since I lodged this question, so that's peculiar.
I had checked permissions and installation issues, but I have another occurrence of Kirby running on another web-server from the same provider with no issues. Again, it was an error that was only occurring on some back-end pages and not others.
I will come back if it occurs again. Thanks for your suggestions thus far!