Temperature and angular resolved measurements of the upper critical field Bc2 of UPd2Al3 thin films are presented and compared with new theoretical calculations for weak-coupling s- and d-wave superconductors in the clean limit. In order to take account of the observed anisotropy of the initial slopes near Tc, B?c2 = ?4.9 T/K for Bc and B?c2 = ?5.8 T/K for Bc, and the crossing of the phase boundaries in the temperature dependence of Bc2 near T* ? 0.75 T/Tc, the calculations include anisotropic orbital and paramagnetic pair breaking. Recent speculations upon the occurence of a non-uniform superconducting ?Fulde-Ferrell-Larkin-Ovchinnikov?-state are also considered and incorporated in the calculations. In the region around T* a minimum between Bc and Bc in the out-of-plane angular dependence of the upper critical field Bc2(?) is found experimentally. This cannot be explained by an s-wave order parameter and reduces drastically the possible d-wave functions. To explain the overall data, a mixing of s-wave and a particular d-wave order parameter is proposed.