This is a regression introduced in Emacs 26.1 - no problem before that.

emacs -Q

1. In a Dired buffer (directory `toto', say) create a subdir `foo', mark
two files in `toto', and then use `C' to copy those two files from `toto' to
`foo'.

2. Use `M' with `a-w' to make the copies in `foo' read-only.

3. Turn on `debug-on-error'.

4. In `toto', use `C' again to try to copy the same marked files.

Let's say that the first file to copy is `a'.  You see this prompt:

   Overwrite `/the/path/to/toto/foo/a'? [Type yn!q or C-h]

Use `C-h' to try to see the key help.  This happens:

`help-form-show' raises an error: (void-variable to).

It seems like the "to" in the prompt is being evaluated as a variable.
Perhaps there is a missing quote somewhere.

You can no longer use any keys at this point, including mouse actions.
(Except you can use `q' to quit the debugger.)

Because I cannot use keys at that point I cannot select the text in the debugger to show it. Instead, see attached screenshot (file names redacted).

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''