> Maybe something has changed in Office 356?
Might well be, or maybe OLE is blocked through one of the many OLE-related security patches that MS has released. However, have you tried if any other usable OLE servers are available in the pull-down menu (in the filetype config) where it currently says "Excel.sheet.16"?
In the distant past (I don't know if it's still true) Office programs had a 'Save Preview Picture' option in File | Properties. When a document had such a preview image embedded, ThumbsPlus could read it when you had 'Load file using' set to 'Internal'. I'm only mentioning this for completeness sake - even if it still works, it won't be a satisfying solution for most people because few documents will contain a preview.