The curvilinear element used in HFSS Eigenmode solver leads to this message "failure in matrix solver".
Curvilinear elements can cause problems in the case of complex geometry and to find the nearest true surface might not be unique. To deal with this issue, it is recommended to increase the number of segmentation of the curvilinear element and it will work fine.
Usually, Eigenmode problems are very sensitive to the mesh quality. Mesh quality also depends on the aspect ratio. Decreasing the aspect ratio can result in a better quality mesh.