We have a couple of users in our network who have started getting the dreaded windows security window popping up to ask for their password credentials when starting up Outlook.  What is really weird is that it will cycle through letters and special characters for the first character in domain and the username.  

For example, out set up is domain\firstname.lastname.   However, we had a case where it waszomain\birstname.lastname  and even a case where it was omain\irstname.lastname.  

Users affected are Windows 7 machines running Office 2013. Going to Control Panel>Credentials and there are no regkeys under Autodiscover to reset. 

This is very bizarre, and I haven't been able to find another case like this.  Any help is greatly appreciated!

Hi,

There are several reasons that may cause Outlook to prompt for credentials:

• Outlook is configured to prompt you for credentials
• Incorrect password cached in credential storage
• Required Authentication Settings for outgoing server and incoming server
• Outlook Anywhere is not configured to use NTLM Authentication
• Corrupt Outlook profile
• Slow or unstable network connection
• Antivirus programs
• Shared calendars

Please have a look at the following TechNet article and check if any of the suggestion may fix your issue:

https://social.technet.microsoft.com/Forums/exchange/en-US/bcd2d9c2-1a1b-4446-bf32-69fee8cdf11b/why-does-outlook-keep-prompting-for-password?forum=outlook

Please let me know the result.

Regards,

Steve Fan
TechNet Community Support

Hi Steve, 

Thanks for providing that information.  I am actually more concerned about the cycling of letters vs the credential prompt.  Any idea why some users will get the credential prompt with the correct info vs getting these randomly generated letters replacing the first letter of the username and the domain name?