Today we had the exact same problem in our environment, so I thought I'd share our findings in case somebody else faces the same situation in the future:
We found that the problem was caused by the renewal of our Blue Prism license. Even if we had already installed a new working license on the server, the runtime resource was still using the old license information. This means that even if the server tried to create a session on the resource, the resource was unable to access any of the objects (even internal ones, like the credentials VBO) because that's disallowed while being ""unlicensed"" (just like interactive clients disable most of the tabs when a valid license is not found).
The solution was to simply restart the runtime resource (the application, not the computer), so it would fetch the new license information from the server.
The need to restart the resources is actually written on the Knowledge Base (https://blueprism.knowledgeowl.com/help/new-license-installation-proble…), but the error you get if you forget to do so is incredibly misleading, as can be seen by reading Blue Prism's John Carter's surprised reaction on one of the previous comments.
My feedback to Blue Prism: I do not know if the newer BP version (6.0) checks for the resource's license status before trying to physically create a session, but I think that doing so might produce a more ""readable"" error message. Also, wouldn't it be possible to just ""hot swap"" licenses on all connected resources once a new version is installed? Restarting them might not be viable in busy environments with a tight schedule or when there's a very large number of them.
