Dear Aleks,
thank you for helpful suggestoins! The issue of divergence slipped past my attention, as a very similar calculation -- started from the same geometry, with the same values of FDOCCUP and SPINLOCK, but without NODIIS and EIGSHIFT -- converged in just 31 SCF cycles.
I tested your suggestion of adding to this input FMIXING (I went with 65%) and increasing TOLINTEG to 9 9 9 11 20, but it still diverges:
CYC 0 ETOT(AU) -3.272838921072E+04 DETOT -3.27E+04 tst 0.00E+00 PX 1.00E+00
CYC 1 ETOT(AU) -3.265248439176E+04 DETOT 7.59E+01 tst 0.00E+00 PX 1.00E+00
CYC 2 ETOT(AU) -3.264532086387E+04 DETOT 7.16E+00 tst 3.64E-01 PX 9.98E-01
CYC 3 ETOT(AU) -3.265874325232E+04 DETOT -1.34E+01 tst 1.16E+00 PX 2.45E+00
CYC 4 ETOT(AU) -3.264891945690E+04 DETOT 9.82E+00 tst 7.87E-01 PX 2.27E+00
CYC 5 ETOT(AU) -3.254860144024E+04 DETOT 1.00E+02 tst 4.49E-01 PX 2.71E+01
CYC 6 ETOT(AU) -3.058859793674E+04 DETOT 1.96E+03 tst 3.20E+00 PX 3.32E+02
CYC 7 ETOT(AU) -2.155342398855E+04 DETOT 9.04E+03 tst 3.59E+03 PX 1.30E+03
CYC 8 ETOT(AU) -1.061409920226E+04 DETOT 1.09E+04 tst 6.90E+03 PX 6.97E+02
CYC 9 ETOT(AU) -1.534233758958E+04 DETOT -4.73E+03 tst 6.48E+03 PX 6.91E+02
CYC 10 ETOT(AU) -2.109155329839E+04 DETOT -5.75E+03 tst 2.31E+04 PX 5.40E+02
...
CYC 20 ETOT(AU) -2.714221021396E+04 DETOT 1.99E+01 tst 1.56E+02 PX 3.07E+02
CYC 21 ETOT(AU) -2.682694963926E+04 DETOT 3.15E+02 tst 7.09E+01 PX 2.97E+02
I guess, there is just no simple way of forcing a particular spin configuration AND a particular charge state to an ion 