The error cited in your original message is reported in the internals of the MLSync object.
The IsValid() check only matters if you are displaying the MobiLink Synchronization Status window. If that window IsValid(), the MLSync application object will trigger an event on that Window to log status information. It does nothing specific to the sync so the fact that it is or is not valid should not matter.
I have tried a number of 'connectivity' type issues - ML server down, bad client connection info - and am not able to reproduce this error.
I would suggest that you might need PowerBuilder help for this issue and more specifically, help from someone with knowledge of the internals.