Unwilling server: Can't unlock AD account in code

When I try to unlock an AD account using my own C# program, I get the following error:

System.DirectoryServices.DirectoryServicesCOMException (0x80072035): The server is unwilling to process the request.

This is the code I use to unlock the account:

// "ldap" is an instance of my own class for accessing an LDAP server

using (DirectoryEntry entry = ldap.GetEntry(objectGuid))
{
    entry.InvokeSet("lockouttime", 0);

    // I also tried:
    entry.Properties["lockouttime"].Clear();

    entry.CommitChanges();
}

I use this software in multiple domains and only get this error in one of them and I can't figure out what the difference is. When I use dsa.msc to unlock the account, everything works fine.

The error also happens with different user objects, but both versions (Clear and InvokeSet) work in other environments. Can anyone give me a hint?

P.S.: I use domain admin credentials to access the LDAP server.

Try this example:

   public void Unlock(string userDn)
    {
        try
        {
            DirectoryEntry uEntry = new DirectoryEntry(userDn);
            uEntry.Properties["LockOutTime"].Value = 0; //unlock account

            uEntry.CommitChanges(); //may not be needed but adding it anyways

            uEntry.Close();
        }
        catch (System.DirectoryServices.DirectoryServicesCOMException E)
        {
            //DoSomethingWith --> E.Message.ToString();

        }
}