For a customer we have created a number of Jira Service Desk projects for internal use. They have worked fine until recently.
But as the company didn't use Service Desk in their support operations anymore, they discontinued that part of the subscription.
We realized that we need to direct employees to the standard Jira interface instead of the Service Desk interface for those projects. Not a big issue - we assumed.
However, it seems like the projects have started to behave a bit strange. For instance, I'm not able to add people to the projects anymore.

I find this error message to be pretty strange. It comes up whatever I write, and btj are the initials of a person who is already a member of several projects on this site.
Another strange thing happening is that one of the projects I can't open anymore. Whatever I click on I always end up in the _Project Settings -> Details_ screen. If I click on _Back to project_ it just reloads the Details screen.
Does anybody have a clue to what's going on?
Is it not possible to continue to use a Jira project created as a Service Desk project, if the Service Desk subscription is cancelled?
I certainly don't hope that's the case, but if it is, how do we migrate all the issues to new projects then?
Thank you for any suggestions.