This error occurs in every buffer (except Lisp buffers)
No items suitable for a index found in this buffer
whenever I apply Imenu-list-minor-mode.
That message tells you that Imenu support has not been defined for the mode of that buffer. That's all. Are you sure that you don't see Imenu support in some modes, such as c-mode?
If you truly do not see Imenu support anywhere except Lisp mode then recursively bisect your init file to find the culprit. You can use command comment-region to comment out 1/2, then 3/4, 7/8,... of the file, to narrow it down using binary search. C-u with comment-region uncomments the region.