I think you have the "Cenote Lockpoint" app (used to be Aresnale lockpoint) which is capable of locking attachments dynamically and controlling access to them. The error you've got is quite generic, but I would ask your space admins to check your permissions, and then ask the Confluence/system admins to read the log files at the time you try to edit, as that will give you the full error that you'll need to debug this problem.
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