Does the user in question have more than 1 group attributed to them? Do any of those other groups have access? If so, this would cause that.
Otherwise, we would need the forum name and an example user's display name (or ID) to investigate. Along with access:
We would need to look further into this for you, however the access details on file appear to be incorrect or missing. Could you please update these details by visiting your client area, selecting the relevant purchase, then clicking "Review/Update Access Information" under the "Stored Access Information" section.
We look forward to further assisting you.