SourceTree does not start ANY MergeTool - unless which one I configure (BeyondCompare, KSIff3, TortoiseMerge). I even tried to configure KDiff3 as "Custom" MergeTool - without any success ...

What happens when starting a MergeTool from Sourcetree:

• Infobox pops up, saying external mergeTools is gonna be launched
• The cursor changes to a dotted circle
• The cursor changes back to normal
• That's it - nothing else

Hint:
Within my Sourcecode-Folder a set of Files named basename_LOCAL*, basename_REMOTE*, basename_BASE* and basename_BACKUP* are created. Those files are not removed after MergeTool startup failure