your password cannot be changed. please contact your administrator for assistance RDWEB 2012 R2

Were you able to solve this?  I am getting the same error in my log.  It appears that password resets are working if they meet the criteria (min age, complexity, etc...).  If the old password is incorrect or the new one doesn't meet criteria we get the "Unhandled error in changing password" in the RDWeb log.  Here's the exact message:  

w3wp.exe Information 0 2016/11/02 07:41:25 [Verbose] 3 Page Requested : password.aspx, Request Type : POST. w3wp.exe Information 0 2016/11/02 07:41:25 [Info] 3 ExtractInfoFromForm returning : True. w3wp.exe Information 0 2016/11/02 07:41:25 [Info] 3 Error in changing password. DN: CN= ,OU=,DC=,DC=lan, ErrorCode: 3221225580, Exception: System.Runtime.InteropServices.COMException (0xC000006C): Exception from HRESULT: 0xC000006C w3wp.exe Information 0 2016/11/02 07:41:26 [Info] 3 Unhandled error in changing password. DN: CN=,OU=,DC=,DC=lan, Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Runtime.InteropServices.COMException: Exception from HRESULT: 0xC000006C    --- End of inner exception stack trace ---    at System.DirectoryServices.DirectoryEntry.Invoke(String methodName, Object[] args)    at Microsoft.TerminalServices.Publishing.Portal.FormAuthentication.TSDomainFormsAuthentication.DoPasswordChangeRequest(Object source)

In the UI the user sees the message "Your password cannot be changed. Please contact your administrator for assistance." instead of the descriptive message telling the user what is wrong. 

Any help is appreciated.