Wondering if the OP (or anyone else) has come up with a solution to this. I am having the same issue with CS4 templates in CS5.

The templates have gone through the IDML “rinse cycle” — export as IDML from CS4, open and save from CS5.

When I select “Insert Pages” from the Pages panel and attempt to insert more than two pages based on a master page, InDesign crashes every time.

It does not crash when the [None] template is selected, and it does not crash when master page spreads are dragged manually to the pages panel to add pages to the document. (But that's a cumbersome way to work.) It also does not crash when new pages are added to a freshly created CS5 document. I haven't tested any CS5 templates yet.

CS5 preferences have been rebuilt. This is an upgrade from CS4, based on a clean install of Snow Leopard.

Any ideas? I can't afford to rebuild all of my legacy templates from scratch in CS5.

SB