I did as recommended. Unfortunately this did not solved the problem. I found out that in pmatin.f in line 106 the material number instead of the user element number is given to vinput(). For now I just made a little workaround because I dont know in which variable the real user number is saved.
if(pcomp(tx(1),'user',4)) then ! Get name as 'user ma'
WRITE(tx(2), '(A)') '7'
errck = vinput(tx(2),15,uelnum,1)
iel = nint(uelnum(1))
else