This workflow errors out occasionally and I can not determine why. Is there any way to get more details? Perhaps any details on what data is actually being pulled from the SQL table?
in your workflow settings page you have the option to "Enable verbose logging". Check this and re-publish the workflow. After the next time it runs, you can go to your graphical workflow overview and click on your execute sql action. A popup window will open and provide a detailed view about before and after values of your action. This way you can see whatever is happening to your variable values inside any action of your workflow.
If this option is not available in your workflow settings, it needs to be activated on farm level first. This can be done in the nintex settings inside central administration.
What type of data are the columns you are trying to update? Is any of the fields updated correctly? If not, can you test the workflow by performing the set field value actions one by one just to see which field throws the error?
Retrieving data ...