Always get "Oops, you've made a malformed request" in installation

I tried to update an old 1.x version by deinstallaing and installing the current 2.x version. Whenever i reach the item "License aggreement" in the installation program and click "use an existing account" i get the message "oops..." after entering "ehoesch@web.de" as login name and password. I tried several older versions without any improvement. I dropped all cookie from all browsers. no effect.
If I click on "Go to my Atlassian" I can login without any problems but i can not see how this can lead to any progress in the installation process. Was a perfect tool on mac and the old 1.x was superb on windows. Now i can not use sourcetree any more.