In Umbraco 6.1.6, we've been able to successfully use ActiveDirectory for our BackOffice login (by adding the connection string in web.config and modifying umbracoSettings.config). Works great for maybe a few days or weeks. Then we'll get a "server is not operational" YSOD after trying to login, with the text shown below. A stackoverflow article has several suggestions, and within that article, the "Connection pooling" answer seems a good bet, but I don't know how to apply that answer. The only "solution" so far has been to kill the IIS process (or restart the server), then things work fine again. We'd love a better solution. (We use the same type of LDAP connection both on that server and other websites and it works fine.)

[COMException (0x8007203a): The server is not operational.
]
   System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail) +627365
   System.DirectoryServices.DirectoryEntry.Bind() +44
   System.DirectoryServices.DirectoryEntry.get_AdsObject() +42
   System.DirectoryServices.DirectorySearcher.FindAll(Boolean findMoreThanOne) +98
   System.DirectoryServices.DirectorySearcher.FindOne() +44
   System.Web.Security.ActiveDirectoryMembershipProvider.FindUserEntry(DirectoryEntry containerEntry, String filter, SearchScope searchScope, Boolean retrieveSAMAccountName, String& sAMAccountName) +265
   System.Web.Security.ActiveDirectoryMembershipProvider.ValidateUserCore(String username, String password) +1970
   System.Web.Security.ActiveDirectoryMembershipProvider.ValidateUser(String username, String password) +39
   umbraco.cms.presentation.login.Button1_Click(Object sender, EventArgs e) +193
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +155
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3804