For diagnostic reasons: could you try with Kleopatra as well?

Maybe this is relevant:
I only installed gpg, gpgEX and GPA from the GPG4Win package.

Using gpg.exe on the command line works perfect.
I can en- and decrypt files and sign and whatnot.
Only gpgEX is affected.

The crash happen regardless of what I did before. Usually I use Q-Dir as file
browser. When I trigger a gpgex action via context menu in one Q-Dir OR Windows
Explorer the program (the one I used) crashes.

I set GNUPGHOME as a user variable via Windows' Extended System Settings.

I followed Bernhard's suggestion and attached the GpgEX debug log.

Please provide a proper bug report so that we are able to replicate this. For
example, how did you set GNUGHOME, what process have been started before that
etc. A complete run trough on how to exhibit the bug would be a appreciated.

Thanks for asking again, I did not remember that GPGex was missing from the
compendium.

It works similiar to GpgOL, see
http://git.gnupg.org/cgi-
bin/gitweb.cgi?p=gpgex.git;a=blob_plain;f=README;hb=HEAD
and in German:
http://lists.wald.intevation.org/pipermail/gpg4win-users-de/2013-
August/000593.html

Actually, I can't.
The link you provided leads to a site with some general infos.
There is another link into the compendium with infos about
some GPG4Win components but not a single word on gpgex.

So HOW do I provide some more diagnostic output?

Fixed in d3f20e7. Applied patch to gpg4win as well.

In the backtrace gpa_backtrace2.png you can see that the crash happens:
https://bugzilla.gnome.org/show_bug.cgi?id=696232

If we apply the patch it no longer crashes, but as the error handling in that
patch is just to abort, the text is not copied into the clipboard and we do not
get an error in GPA.

So still some more debugging why invalid utf-8 data is sent from gpa.
As the data worked on comes from GPA and "should" be valid utf-8

Backtrace attached. Sorry that it is an image but windows was blocking me from
copying the gdb output to the clipboard.

Libgcrypt does not support 64 bit Windows yet. In particular do not use it even
if it would build and run fine. MSVC is not a supported build platform anyway.