I'm posting this for the benefit of anyone else who faces this issue. In setting up the integration between NAV 2016 and CRM 2016, we started getting errors whenever someone tried to open the CRM Sales Order List page in our test environment. The errors were different - I got an error that said the parameter could not be null, parameter name: key. This would cause the page and ribbon to disappear, but the rest of the client application would still be fine. I could exit the page and continue working in the client. Two of my colleagues who tested it got an error that was more generic and caused the entire application to close. This only occurred in the test database - that page could be accessed in the production database just fine.
After a lot of testing and banging my head, I finally realized that the culprit was a field that had been added to the CRM Salesorder table. Apparently, when you add a field to that table (and I would assume it holds true for other integration tables, as well,) there are some properties of that field that need to be set that are not actually displayed in the development environment. You have to export the table as text to add those properties, which include "ExternalName" and "ExternalType."
After the field had been updated with those properties and re-imported, the CRM Sales Order List page could be opened without any issue.
*This post is locked for comments