When I edit the Canada Post shipping plugin a new warehouse is created whenever I select "Save" or "Save & Close". With "Save", the new warehouse - with empty fields - appears immediately; with "Save & Close" the new warehouse appears when the plugin is opened again for editing. If the new one is not trashed each time, more and more new warehouses will appear. Since it now looks like that this not the cause of CP returning errors at checkout, it is simply a frustrating inconvenience and so far apparently not causing any other issues.
Note: I found this thread while searching for the new warehouse problem, which was first posted by geeksonthebeach in #165462.