Why are 'Insert Cells', 'Delete Sheet' and other options greyed out?

When I select Sheet / Insert Cells, the options ‘Shift cells down’ and ‘Shift cells right’ are greyed out. The ‘Entire row’ and ‘Entire column’ options are available.
I also see that a number of options on the Sheet menu are greyed out: ‘Delete Sheet’, Show Sheet’, ‘Move or Copy Sheet’.
Which sounds as if the sheet is protected, but the ‘Protect Sheet’ and ‘Protect Spreadsheet Structure’ options on the Tools menu are very definitely not set. I’ve tried setting them and unsetting them - no change.
What can be causing this?
I should add that this is a rather large document- 36 sheets with 50k cells and 15k formulas.

You probably have change tracking activated. Deactivate menu Edit → Track Changes… → Record, or Shift+Ctrl+C.

That’s it! Many thanks.