Everything had been fine until this point, but after creating a simple example approval workflow, I clicked the Submit button on the Purchase Requisition Entry window and nothing happened; I tried again through the navigation list, again without success.
I promised to figure out the issue in the next break and moved onto discussing other ways of interacting and then called the break.
It occurred to me that this might be another manifestation of an issue I have encountered previously, but without the visible error message.
I ran the script against the system database which redeploys the .NET Assemblies used by Workflow:
It took a few minutes to run through, but I was then able to approve the requisition without further problem.