Question: Strange error in diff

Hi,

This is my first post here, so I apologize for any errors or breaking of posting etiquette. I encountered a strange problem suddenly tonight when I tried running a Maple worksheet I've run succesfully (several times, without changes) in the past. The problem occured on both Maple 18 and Maple 2015, unfortunately I haven't been able to recreate a minimal working example with the same error, since my other worksheets doesn't show the error even though they contain identical caluclations. The problem also persists between copies of the worksheet.

The error I'm recieving is
"Error, (in TypeTools/ac_var_local) unable to handle expressions as (((D@@2)(phi))(theta(t)))(t)"

It occurs whenever I try to differentiate some (not all) expressions containing the above variable. I have no idea what this error even means, and I can't find anything googling. As said previously, I haven't been able to produce the error so I can't really give a concise short example but the flow is as follows:

g0 := simplify(subs(diff(y(t), t, t) = F+N*v(t), lhs(aby)-lhs(aby_y)), 'size');
->Produces an expression containing (((D@@2)(phi))(theta(t)))(t)
diff(g0,t)
-> Gives Error

U := lhs(Vector([1, 0]).subs(subsVhcError, eom));
->Produces an expression containing (((D@@2)(phi))(theta(t)))(t)
diff(U,t)
-> Does not give error

Sorry that I can't be more descriptive, does anyone know have to sort this out, or might give me some more info on what the Error message might mean and how TypeTools are involved? As stated previously, the error just occured today and only affects one worksheet.

Edit: Link to worksheet with error
transverse_linearization.mw