When I choose a beast race with 1stPE enabled I get these errors:

Choosing argonian race:Model Load Error: Meshes\new_beast\arg_m_body.nif cannot load file in Meshes\new_beast\arg_m_body.nif.
Will use the default object Marker_Error.NIF.
And with khajit race: Model Load Error: Meshes\new_beast\khaj_m_body.nif cannot load file in Meshes\new_beast\khaj_m_body.nif.
Will use the default object Marker_Error.NIF.

Now, fror doubt's sake I've even unpacked MW and expansions bsa files but that folder (Meshes\new_beast) doesn't exist. My first thought was that it had to do with Liztail New Beasts, but I have Better Beasts and I've chosen the 1stPE compatibility esp for BB. So I really don't know why MW is searching for that folder.

Disabling Darknut's mod the problem disappears and it seems that changing load order doesn't help at all.

Anyone else has encountered this problem?

Read the readme ... this is a installation issue .... the better beast files are not where they are supposed to be or have been renamed to something other than the original names

Maybe in the past you dealt with people that posted their problems here without even reading readmes (and I think it often happens), but do you take for granted that noone reads the readmes? If it's so it's not my problem, and when you are going to answer with "read the readmes...", well please stop yourself and don't reply at all.

So, as you may have guessed I've read both readmes. As for Better Beasts, I've put "Slof's Better Beasts b.esp" and "Slof's BB neck fix" (I use BH) in this order, and extracted AS folders for meshes and textures (into their respective folders). Concerning 1stPE, I've extracted nif and kf files into the meshes directory, and extracted DN_1st_Person_Armor.esp/DN_1st_Person_Bodies-AS.esp (in this order) into Data Files directory. No files renamed (why should I have to???)

So I think I've well followed readmes instructions, but these mods don't work well together for me. It may be that I've done something wrong and I can't figure out what. It also may be that my computer doesn't like them together (very likely).

Your right the vast majorities of issues people have is either from not reading the readme at all or not following what the readme says to do, so if your not one of those ... I'm sorry.

I know this mod works with both beast mods .. I have used them both .. a lot .... try reinstalling both BB & 1st person enhanced.
The error MW is giving you is telling you what is wrong IE: "Model Load Error: Meshes\new_beast\arg_m_body.nif cannot load file in Meshes\new_beast\arg_m_body.nif."

MW can't find the meshes because they are not where they are suppose to be. IE: MW is looking here -> Meshes\new_beast\ ... but the files are not there ... & that's where they need to be