I downloaded the setup "SourceTreeSetup_1.6.23.exe" from the atlassian website.
The installation process with administrator rights works fine (shows no error).
I configured the openSSH Client and my id_rsa file.
If I start the software now I'll get an error:
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Ein Fehler ist aufgetreten
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'ssh-agent' ist fehlgeschlagen mit Code -1: System.Exception: Unable to to start 'C:\Users\administrator\AppData\Local\Atlassian\SourceTree\git_local\bin\ssh-agent.exe' check the git installation.
()
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OK
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If I look in the folder C:\Users\administrator\AppData\Local\Atlassian\SourceTree\git_local\bin\ there's no ssh-agent.exe.

Workaround:
I converted my OpenSSH key with the Putty Key Generator into a Putty keypair. Then the error is gone.