"STOCKHISTORY function is not working: If you are using Office 365, try performing an Online Repair by right-clicking the Windows Start button, selecting “Apps and Features” on the pop-up menu, finding and clicking your Microsoft Office 365 application in the list, selecting Modify, then “Online Repair” and following the onscreen instructions." Go for online repair rather than quick fix.
Takes a while depending on download speed. There must have been a glitch in a recent update, which this fixes. Some settings change in minor ways (e.g. need to put Excel back on the taskbar). After the fix, most cells updated straight away, with all completed after 20 minutes, and the "BLOCKED" message disappeared.
This has made me think about my backup strategy, because all my backups were affected in the same way. Need to find an easy way to freeze values in the backups.