I also need to give credit to the Office 365 support team over this. They were very quick in their response to my support incident, but I was quicker!
Whilst working on an Office 365 site for a customer today I had a moment of blind panic. The site is using custom branding and I was uploading a new version of the master page to the site when things went badly wrong. The upload appeared to finish OK but the dialog that was shown post upload was not the usual content type/fill in the fields form, but a plain white box. I left it for a few minutes but nothing changed. Unperturbed, I returned to the mater page gallery… Except I couldn’t. All I got was a white page. No errors, nothing. No pages worked at all – no settings pages, no content pages, nothing at all.