Answer:
The message means that your structural system is numerically unstable. Such instability is often caused by incorrect support conditions or hinge definitions in RF‑/FE‑LTB. Therefore, check the degrees of freedom of the continuous member in Window 1.4 Nodal Supports and Window 1.7 Member Hinges.
Background
Both when selecting the imperfections and in the actual calculation, the critical load factor is determined first. It is then used to determine the critical load of the system. This is characterized by the fact that either the determinant of the stiffness matrix becomes zero, or very large deformations occur in the calculation for small load increments.
