Fail to trim/ intersection/ boolean

pretty clean object. surprised that it actually fails for no reason. temp.3dm (304.4 KB)

Actually, it’s not. It’s invalid (bad object). Use Check to see:

Rhino polysurface object is not valid.

brep.m_L[21] loop is not valid.
end of brep.m_T[loop.m_ti[0]=102]=(118.03,266.977) and start
of brep.m_T[loop.m_ti[1]=103]=(30.7299,199.5) do not match.
brep.m_F[21] face is not valid.
brep.m_L[face.m_li[0]=21] is not valid.
ON_Brep.m_F[21] is invalid.

If you extract the one bad surface - you can already see it’s bad by a trim line that goes into space - delete it and then Cap the object to close, the Split will work.