We have measured the current-phase relationship I(varphi) of symmetric 45 degrees YBa2Cu3O7-x grain boundary Josephson junctions. Substantial deviations of the Josephson current from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature T, a local minimum at a temperature T*. (ii) At T approximately T*, the first harmonic of I(phi) changes sign. (iii) For T
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