Launch external merge tool when resolving merge conflicts caused by stash pop: files not found

[wrd90]

I am trying to unstash some files, one of the files causes merge conflicts, which I am trying to resolve using my external merge tool Beyond Compare. I have never had to resolve merge conflicts due to unstashing using SourceTree, so I might be doing something wrong. 

Here's what I did:

1. I applied the stash, which reported errors due to a merge conflict.
2. I went back to my working copy's file status
3. It shows the conflicted file once in "staged files" and once in "unstaged files", both with an exclamation mark indicating conflicts.
4. The version in "staged files" shows no changes
5. I right-clicked on either of these versions and chose resolve conflicts/launch external merge tool
6. My diff tool launched, but did not find these files "<whatever>_LOCAL_21148.tex" "<whatever>_BASE_21148.tex" etc.
7. But it creates new files named .merge_file_<some_hex_number> which as far as I can see contain the base, local, and remote versions. 

Can someone point me to what's going wrong here?

Edit:

The same happens when I use git mergetool

Best,

David