Mercurial credentials do not work

OS: Win 10 Pro Build 1607
Using SourceTree's embedded Mercurial 3.7.3 with pretty much default settings.
Using my SourceTree installation for managing multiple repos (Git and Mercurial).

Trying to use mercurial functions which require authentication for remote repo spawns infinite "Git Credential Manager For Windows" popup windows (see screenshot). Git is in no way related to this mercurial repo. With "infinite", i mean that on closing that popup or filling it with correct credentials and confirming, the only thing that happens is that another one opens, as long as the underying operations is trying to complete, over and over again. (Can be stopped by for example closing the "pull window", which of course cancels the pull).

The issue occured since i started to use Sourcetree for Mercurial with a normal (private) remote Mercurial site (which is not integrated in Sourcetree like Bitbucket).
Before that, i used the same local repository, but only used my integrated BitBucket account as remote, which worked fine.

The issue occurs whenever Sourcetree tries to access any functionality that has something to do with the remote repo (push, pull).
This means, that the remote cannot be used as default remote, because in that case Sourcetree seems to pull on opening Sourcetree which results in unavoidable infinite credential managers popus which can only be get rid of by closing Sourcetree completly.

The remote repo url looks like this:
https://team.iisys.de/mercurial/va/heimdall/cubbles-generator\\
The other remote for the same local repo on Bitbucket looks like this:
https://Katharsas@bitbucket.org/Katharsas/heimdallgenerator\\
Trying to do the same mercurial operations (push, pull) with the console works perfectly, the console first asks for credentials and after entering those completes the operation as expected.

Expected behaviour:
Should work like console or at least throw a meaningful error.