I am using Emacs from HEAD on Windows

I tried to reproduce the behaviour with emacs -Q but it still remains
inconsistent.

When I rgrep and there are few results (< 15) , I can enter grep buffer
pres 'e' to enter grep-edit mode, get out of it with C-c C-c, re-enter with
'e' and so on.

When there are more results, (between 15 and 70)  I still can enter with
'e' get out with C-c C-c but when I try to re-enter I get the error message
Wrong type argument: stringp, nil
The buffer later although still in grp-mode does not behave any longer as a
grep mode, 'g' or 'e' results in the error Buffer is read only : #<buffer
*grep*>

When there are many results, I get the error message Wrong type argument:
stringp, nil already on the first invocation attempt.

Thew boundaries are somewhat moving but at least I have the feeling the
more matches are, the more frequently I run into the

Wrong type argument: stringp, nil

already at first try.