After much digging into the WSS logs message that I believe is the reason why documents cannot be checked out or deleted:
“Error when trying to get trusted forests and domains. Exception message: The specified domain either does not exist or could not be contacted. Name: "ad.mpc.com" , callstack: at System.DirectoryServices.ActiveDirectory.Locator.GetDomainControllerInfo(String computerName, String domainName, String siteName, Int64 flags) at System.DirectoryServices.ActiveDirectory.Utils.GetPolicyServerName(DirectoryContext context, Boolean isForest, Boolean needPdc, String source) at System.DirectoryServices.ActiveDirectory.Forest.GetTrustsHelper(String targetForestName) at System.DirectoryServices.ActiveDirectory.Forest.GetAllTrustRelationships() at Microsoft.SharePoint.Utilities.SPUserUtility.GetTrustedDomains(List`1 trustedForestNames, List`1 trustedDomainNames)
Please let me know if you have seen this kind of errors and how can it be fixed.