Git unable to fetch, pull or clone using https

I am having problems fetching, pulling and cloning from a git repository. I get the following error message

git -c diff.mnemonicprefix=false -c core.quotepath=false fetch origin
fatal: unable to access 'https://name@domain.tld/scm/project/projectname.git/': error:14077102:SSL routines:SSL23_GET_SERVER_HELLO:unsupported protocol

Server is a self hosted BitBucket server and Client is Sourcetree 1.8.3.0. Git is 2.6.1, but getting the same error with 2.7.2

Client Machine is Windows 10.

The same configuration works on a Mac. Also with TortoiseGit instead of Sourcetree give the error. So I suspect that Win is causing the problem.

How can I find out, what the real issue is? The error message is not very helpful.

Update:

this is what I get, when I perform a git pull from the command line

C:\0.GIT\project>set GIT_TRACE=1
C:\0.GIT\project>set GIT_CURL_VERBOSE=1

C:\0.GIT\project>git pull
08:48:43.730071 git.c:348               trace: built-in: git 'pull'
08:48:43.734740 run-command.c:343       trace: run_command: 'fetch' '--update-head-ok'
08:48:43.974726 git.c:348               trace: built-in: git 'fetch' '--update-head-ok'
08:48:43.988735 run-command.c:343       trace: run_command: 'git-remote-https' 'origin' 'https://ulrich@server.domain.tld/project.git'
* Couldn't find host server.domain.tld in the _netrc file; using defaults
* timeout on name lookup is not supported
*   Trying 81.xxx.xxx.xxx ...
* Connected to server.domain.tld (81.xxx.xxx.xxx) port 443 (#0)
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
* successfully set certificate verify locations:
*   CAfile: C:/Program Files/Git/mingw64/ssl/certs/ca-bundle.crt
  CApath: none
* error:14077102:SSL routines:SSL23_GET_SERVER_HELLO:unsupported protocol
* Closing connection 0
fatal: unable to access 'https://ulrich@server.domain.tld/project.git/': error:14077102:SSL routines:SSL23_GET_SERVER_HELLO:unsupported protocol

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