Yes, I checked read-only mode, and it was not on. Read-only correctly prevents me from entering data, whereas with this problem, I could enter data, it just wouldn't be saved.
However, I have now reinstalled the Notebook software completely, and that seems to have solved the problem. Even the test file I mentioned now works properly. However, I'll keep an eye out in future in case it happens again, and if there's anything else I can do to help you find the cause, feel free to let me know.