Usurper
Erasmus
User
Error
(Uff! Finally I was able to break the evil ending cycle.)
The forbidden endings:
*ea, *er, *ia, *fy, *ic, *ky, *ly, *ll, *on, *ry, *ty, *ud, *um, *us, *xo (for having a small set of options) and all the other two consonant endings that don't start a word.