You do not enter the debugger from
--debug-init in this case because the message is a warning, not an error message. IOW, in spite of the text in the warning message, which speaks of an "error", from Emacs's point of view, an error was not raised. (Or at least not an error that is respected by
--debug-init -- see
Suggestions for debugging:
Recursively bisect your init file, to narrow down where the problem is. Do this by commenting-out 1/2 of it, then 3/4 of it, then 7/8, etc. You can use
comment-region, with and without a plain prefix arg (
C-u), to comment and uncomment regions of text.
Add calls to
message in your init file, to print information to let you know where the error occurs.
Use #1 especially if your init file is complex or it loads other libraries. #2 might suffice if your init file is simple and short.