Microsoft has encountered an issue that could derail customers' attempts to migrate from Windows Server 2003.
The problem, revealed in a Redmond blog post last week, arises when running both Server 2003 and Server 2012 R2 domain controllers serving the same domain - a typical process during a migration.
Users who do this can find that they can't log in to the servers because the Kerberos authentication fails intermittently.
Microsoft admitted it has been "getting a few calls" about this recently, adding that "in the case of the customers that called us, the login issues were actually preventing them from being able to complete their migration to Windows Server 2012 R2".
That's a big issue with Redmond due to retire support for Server 2003 on July 14, 2015, especially with many customers yet to upgrade to a newer operating system like Server 2012.
A Cloud Industry Forum survey of 250 IT decision makers earlier this month found that 53% still run Server 2003.
Redmond said in the blog post that it would be a while before it is able to fix the issue: "We are working on a hotfix for this issue, but it's going to take us some time to get it out to you."
In the meantime, it has issued a series of instructions for a rather unwieldy workaround, found in the blog post.