The board for our next-project was visible yesterday, but as of sometime this morning (~9am BST), this message started appearing.

We have over 5000 issues in the project, but the board should only be including perhaps 100. We are unable to use the software at present due to this issue.
We have attempted to move issues out of this project into an archive one in the hope that we had just crept over some limit, but we have now without doubt removed many more than were added in even the past week, but the problem persists.
The only other change we made recently was to add a new status to the board. No items yet have this status. It's not now possible to access the board to edit its configuration to remove this status.
Is this a defect in the software, or do we have a broken configuration somehow?