Hmm, that error does help a bit to understand what might be going on. Is there an attachment in that project that might be a bit different in some way? Eg. a file link or something? A folder? Web archive?
Could you perhaps even export the project and send it to me at firstname.lastname@example.org?
(I guess if you already deleted the attachments, it may not help me find out what the problem is.
To get things working, you would probably be best to remove any suspect attachments from the project, then add a new project, and drag the notes over to that. I have a feeling the project you have now will continue to have this problem, so better to create a new project.