Restart both Internet Explorer and Lync to check whether the problem is resolved.If your organization requires you to enter specific information for the proxy server or an automatic configuration script, contact your network administrator. Make sure that the Automatically detect settings option is selected.On the Tools menu, click Internet Options, click Connections, and then click LAN Settings.

To resolve this problem, verify that the user has the correct proxy configured in Internet Explorer. If a firewall or a proxy is blocking Lync from connecting to EWS, you may experience symptoms such as repeated credential requests, stale Address Book Service (ABS), and intermittent Free/Busy presence issues. If Lync continues to prompt you for credentials after it has done this several times, there's probably an issue with Outlook or with the Exchange services. Lync generally prompts you for credentials only after you're signed in and when it must connect to an external service such as the Microsoft Exchange Free/Busy service or the Exchange Calendar service. This issue occurs for multiple reasons, and in certain scenarios, it is the expected behavior.

Calendar and Free/Busy information from Outlook.

You may be prompted for credentials when Lync tries to connect to any of the following services:

The message that you receive may vary, depending on which service Lync is trying to connect to.

Skype for business enter credentials