LibreOffice 3.4 RC2
Open a new spreadsheet;
go to Tools > Protect document > Document;
Type a password and click OK;
The “Insert Sheet” context menu item will disappear but you can insert a new sheet using the “plus” signal in the right side of the tab sheets.
I'm working on it and I think I already found the problem. I think it will be fixed in 3.4.1.
committed Markus' nice patch for 3.4.1 - thanks guys ! :-)
RC2 is bit by bit identical with release version, so separate items in the version picker are useless. Changes have been discussed with Michael Meeks.