A while back I came across some very recently invented language that aim to have a simple, uniform, non-contradictory grammar, but I have completely forgotten the name. It's not Esperanto. I'm sorry I can't provide much more details, but I did try searching online for various keywords without any luck.
I suppose what you mean by "non-contradictory grammar" falls under the category of so-called logical languages, which try to establish a strictly systematic and, well, logical grammar without unpredictable irregularities.
The most well-known one is Loglan with derivates Lojban and Ceqli.
Another, indepedently developed logical language is Ithkuil with derivate Ilaksh.
What you are looking for might also be a philosophical language; however these usually put more focus on a systematic lexicon (using an elaborate ontology and as close as possible 1:1 mapping between form and meaning) than on grammar.
The closest possible candidate would probably be Ars signorum.
For conlangs that attempt a strictly systematic composition of words and sentences from atomic parts, see also this question.