Question: How can this worksheet be uncorrupted?

Circular_gears_meshing_blunt_teeth_shape_from_involute_of_circle.mw

My worksheet has suddenly begun producing the following error message when executing legitimate commands:

Error, (in cos) type `SymbolicInfinity` does not exist

Why has this worksheet suddenly begun issuing these mysterious error messages and how can it be restored to working order?

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