This seems to be due to
https://github.com/emacs-mirror/emacs/commit/dfc850ca0223ffd1d2048657ea6d20c971865d51
specifically (backward-up-list 1 t t) seems to emit that error message:
;; Assume called interactively; don't signal an error.
(user-error "At top level")
Easy reproduction: edit a csharp file, go to some toplevel line (e.g. "using System;" that is pervasive in a lot of C# code) and run c-indent-line-or-region.
By the way: (advice-add 'c-guess-basic-syntax :around #'csharp-guess-basic-syntax)
means that csharp-guess-basic-syntax runs also in all other c-mode derived modes. So this error also happens if you first edit a .cs file and then a normal .c file and you try to, for example, indent a toplevel comment in the C code. Very frustrating.
-Aleksey