I have problems when using the Interface (GAP) material on contacts

The interface material settings with tensile strength, cohesion, and friction coeff. all zero are very likely to cause numerical problems. The friction coefficient is hardly ever lower than 0.1 (maybe except an oiled surface), if you have no better measurement/estimate, 0.3-0.5 are usually not far from reality. Cohesion always has to be greater than (or at least equal to) tensile strength times the friction coefficient, i.e., c => ft mu.
A good starting value for both initial stiffnesses is about 10 times the stiffness of the adjacent elements, i.e., E_concrete / elem_size * 10, e.g., 3.0E+04 MPa / 0.1m * 10 gives 3.e+6 MN/m3 for Knn and Ktt. The residual stiffness values for numerical purposes are recommended about 1/100 - 1/1000 of the initial values.
See Section 4.4.3.2 Interface material in ATENA 2D User's manual and 2.6 Interface Material Model in the Theory manual, especially the paragraphs following Fig. 2-38.