Our GitLab CE installation uses for enhanced security of HTTPS communication between GitLab server and browser or GitLab's API consuming application the thing called client certificate, see https://en.wikipedia.org/wiki/Client_certificate. Every user has his own certificate installed on his computer and it's needed to be used when contacting our GitLab server over HTTPS. But when I try to add GitLab CE account to Sourcetree it seems that Sourcetree doesn't recognize this necessity (as webbrowsers do) and tries to contact GitLab server without using this client certificate, so the communication with GitLab server fails.

This is the same issue as it is with macOS version of Sourcetree: https://jira.atlassian.com/browse/SRCTREE-5956