@sofy We don't know what is causing this issue so it's not a question related to an add-on at this stage.

@sofy As per the info here Migrate to wpForo – wpForo – WordPress Forum Plugin your Go2wpForo tool should have migrated attachments, it did not. ...

and the permissions for the forum has Standard access for Registered users  standard-access-registered-users.png

..  registered-user-filetypes.png

..  standard-access-registered-users.png

I can only attach 1 image per reply  user-info.png