I'm still debugging this to find out the exact cause, but since updating to the new 19.1 build, a web app which was working perfectly well has suddenly started throwing an error on starting up which at time of writing I have narrowed down to:

        Send Clear of hoSessionDD
        Set Field_Changed_Value of hoSessionDD Field WebAppSession.SessionKey to sSessionKey  // "iZOoiOFJX0Xd38EyKIVBloVCe0B0TB8CSoWO"
        Set Field_Changed_Value of hoSessionDD Field WebAppSession.RemoteAddress to sRemoteAddress  // "::1"
        Set Field_Changed_Value of hoSessionDD Field WebAppSession.Active to "Y"
        Get Request_Validate of hoSessionDD to iErr  // This returns 90 into iErr
        If (iErr) Begin
            // this should not happen. If it does its a programming error
            Error DFERR_PROGRAM C_$WebAppSesionValidateFailed
            Function_Return ""
        Send Request_Save of hoSessionDD
In the CreateSession function in my subclassed cWebSessionManager. (I am using a different table for WebAppUser, but like I say, it was working fine - for months - until I applied the update, and the subsequent update of the web framework.)

Anybody else seen this, before I waste a couple of hours working out what has gone wrong?