The input grid given for the (Python) test "Loops > Ambiguous loop2":

   X-----+  
   |     |  
   |     |  
   |     |  
   +-----W  

Does not satisfy the guarantee given in the test description:

You are given a grid, which always includes exactly two end-points indicated by X

Moreover, it contains the unspecified character W (a simple typo for X?).