Thank you very much. Problem solved.The generated Thumbnail size is determined by the Core XF Attachments Options "Attachment thumbnail dimensions". What do you have yours set to. The size that XF recommends is 150. If you are using a lower value, increase it to 150 and then rebuild your attachments via the Rebuild Caches system.
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Other reasons could include the author uploading a very small image, which would create a very small thumbnail. If that is the case, the only fix would be to remove that small image attachment and upload one that would be able to generate a thumbnail at least 150 width or 150 height.