External merge tool dosn't launch (e.g. kdiff3 & FileMerge)

I work with SourceTree on MacOS for some years with kdiff3 as external merge tool (and diff tool). For some reason, it suddenly stopped working.

Steps to reproduce

1. Configure kdiff3 (or FileMerge) as external merge tool in SourceTree
   1. Open settings via menu SourceTree -> Settings... -> Diff
   2. Select kdiff3 (or FileMerge) as Visual Diff Tool and as Merge Tool
      
2. Create a git merge conflict
3. Open SourceTree and select a file in conflict in the file status view
4. Select "Resolve Conflicts -> Launch External Merge Tool" from context menu

Observed behavior

• A dialog is opened for about half a second, likely saying, it's launching external merge tool. Then it is closed again.
• Backup files for BASE, LOCAL and REMOTE versions are created.
• No launch tool is opened. Nothing else happens.

Expected behavior

• After creating the backup files, the configured external merge tool dialog must be opened.

Context and Hints

• I checke the external diff tool, too. kdiff3 is correctly opened. Thus, kdiff3 is correctly installed and working.
• I checked the global git config. It seems to be correct. The content is as follows:

[user]
    name = ...
    email = ...
[core]
    excludesfile = /Users/travkin/.gitignore_global
    autocrlf = false
    ignorecase = false
[commit]
    template = /Users/travkin/.stCommitMsg
[difftool "sourcetree"]
    trustExitCode = false
    cmd = /Applications/kdiff3.app/Contents/MacOS/kdiff3 \"$LOCAL\" \"$REMOTE\"
    path = 
[mergetool "sourcetree"]
    trustExitCode = true
    cmd = /Applications/kdiff3.app/Contents/MacOS/kdiff3 \"$BASE\" \"$LOCAL\" \"$REMOTE\" -o \"$MERGED\"

• Trying to close SourceTree and change something in ~/.gitconfig has not result. As soon as I launch SourceTree again, it does re-create its entry for [mergetool "sourcetree"].
• I installed SourceTree and kdiff3 using homebrew.
• I'm using the latest versions of kdiff3 1.10.6 and SourceTree 4.2.7. (263). My OS is macOS Sonoma 14.4.1.
• It seems, this issue is not solved for a long time, since there are multiple more or less similar open tickets (SRCTREE-6683, SRCTREE-2598, SRCTREE-5380, SRCTREE-7266, SRCTREE-7811, SRCTREE-7335, SRCTREE-6308, SRCTREE-1497) and plenty of questions in the Atlassian community, e.g. 1, 2.

I suggest a high ticket priority, since not being able to merge conflicts is a blocker IMHO.