During calculations of a structure for which linear releases have been defined, instabilities connected with the definition of finite elements and their degrees of freedom might occur. The following image shows this situation occurring on a slab.
Robot creates a pinned connection in the place where a linear release has been defined. To avoid instabilities linked with this effect, you should create a special support in which only the rotation about the axis perpendicular to the slab is blocked. This support can be positioned at any place of the slab, provided it does not change the distribution of forces.
After the calculations, you should check if there is a bending moment on the slab edges.