How to debug "Debugger entered--Lisp error: (void-function nil)"

[newsspam5REMOVETHIS@robf.de]

Hi,

I am using "GNU Emacs 22.0.92.1 (i686-pc-linux-gnu, X toolkit)"
and run VM (view mail) in it, but the error comes also with
21.4.1. 

Whenever I enable debug-on-error I get a backtrace with the
following content:

,------------------------------------------
| Debugger entered--Lisp error: (void-function nil)
|  (nil)
`------------------------------------------

How can I debug this to find the source of this problem?

It also happens most of the time when I single step in edebug
mode which makes edebug practically useless.  When running with
debug-on-error set to nil I do not see the error.

Also it seems to be there already for some time as I found other
references when googling for <<<emacs debugger entered--Lisp error:
"(void-function nil)">>>, e.g. http://tinyurl.com/yv6t2j!

Thanks,
Robert