Perform the following steps to refresh Lync cached endpoints:
1. Locate the local application data folder: 
    Windows Vista and Windows 7: %LOCALAPPDATA%\Microsoft\Communicator\<\">This email address is being protected from spambots. You need JavaScript enabled to view it.>\
    Windows XP: %USERPROFILE%\Local Settings\Application Data\Microsoft\Communicator\<\">This email address is being protected from spambots. You need JavaScript enabled to view it.>\
2. Delete the file that is named EndpointConfiguration.CACHE.
3. Restart Lync 2010, and then try to sign in to Lync Online again.

 In the upper-right area of Lync 2010, click the Gear icon to open the Options page. 
2. In the Lync - Options dialog box, click Personal. 
3. Next to the sign-in address, click Advanced. 
4. Make sure that Manual Configuration is selected and that the configuration values are exactly as follows: 
    Internal server name or IP address: sipdir.online.lync.com:443 
    External server name or IP address: sipdir.online.lync.com:443
5. Try to sign in Lync.

On Windows XP, delete the Crypto RSA key container. To do this, follow these steps:

1. Open Windows Explorer and locate the C:\Documents and Settings\<User>\Application Data\Microsoft\Crypto\RSA\ folder.
2. Delete the RSA key subfolder. The name of the RSA key subfolder consists of a long string of numbers and characters. For example:
   
   S-1-5-21-433994307-1646369186-2100375486-500
   
3. Restart the computer, and then try to sign in to Lync Online.

1. Renew the certificate in Windows Certificate Manager. To do this, follow these steps:
   1. Open Windows Certificate Manager. To do this, press Windows + R, type certmgr.msc, and then click OK.
   2. Expand Personal, and then expand Certificates.
   3. Sort by the Issued By column, and then look for a certificate that's issued by Communications Server. 
   4. Verify that the certificate is present and that it isn't expired. 
   5. Delete the certificate and try to sign in to Lync Online. If you can't sign in to Lync Online, go to step 2.
2. If you're running Windows 7, remove the user’s stored credentials in Windows Credential Manager. To do this, follow these steps:
   1. Open Control Panel, and then click Credential Manager.
   2. Locate the set of credentials that's used to connect to Lync Online. 
   3. Expand the set of credentials, and then select Remove from Vault.
   4. Try to sign in to Lync Online again, and then type your new set of credentials.