External diff tool doesn't launch

Since I updated my SourceTree a while ago the "External Diff" is not working anymore. I do not remember the exact version it started to fail.

The internal diff shows the differences but I would like to use my installed WinMerge for diffs.

I checked the <HOME_DIR>\.gitconfig and it contains the following:

[difftool "sourcetree"]
 cmd = 'C:/Program Files (x86)/WinMerge/WinMergeU.exe' $LOCAL $REMOTE
[mergetool "sourcetree"]
 cmd = 'C:/Program Files (x86)/WinMerge/WinMergeU.exe' $LOCAL $REMOTE $MERGED
 trustExitCode = true
[merge]
 tool = sourcetree
[diff]
 guitool = sourcetree 

I opened the "Process Viewer" and executed the "External Diff" command. It reports

Unknown merge tool sourcetree
fatal: external diff died, stopping at <relative_path_to_my_file> 

I stored the "Process Viewer" log to XML, copied out the GIT command line and executed in CMD.exe: it worked as expected and opened WinMerge.

I tried configuring the "External Diff/Merge" section in Tools -> Options using "WinMerge", "Custom" or any other tool: no change

I switched "GIT Version" to "System": no change

I updated the embedded GIT: no change

I switched off using "LibGit2 integration": no change