FEAP UEL - Infinite Displacements?

Hi all,
I am stuck with a problem while starting to learn programming UEL in FEAP.

As a starting point, I programmed Isotropic Linear Elastic element (3D 8 node linear interpolation/shape functions), with one-point integration.
Before running this UEL, I wrote a simple 8 element input file and used FEAP standard Solid element (ELAStic, ISOtropic), the results for this input file were nice.
However when I ran the analysis for this input file with my UEL (USER 04), the displacements were unbelievable (x10^16).

I have printed EVERYTHING in separate output (txt) files, the values in "B" matrix are perfect, as well as the others outputs (including stresses and strains), except displacements (which are very high) in both i.e. FEAP command window (with disp all command) as well as in the Oelas (FEAP output file).

There is no problem with the boundary constraints (sufficient restraints) since the problem run perfectly fine using the standard Solid element of FEAP. Furthermore the analysis doesn't converges (Residual remains constant).

Please guide me about possible sources of errors.
For reference the subroutine UEL and input file are attached. (kindly change the extensions from elmt05.txt to elmt.f and ielas.txt to ielas)