I have recently updated my git (2.7.4.windows.1). Since then pulling from github produces output like this:
$ git pull --rebase
Fatal: TaskCanceledException enco
The credential helper handling changed since the last version that I used. The systemwide gitconfig (as TortoiseGit calls it) contained a section:
[credential]
helper = manager
I think it is some leftover from previous versions. I deleted it and the delay and the error message haven't happened since. Victory! :-)