It's more likely that this activity doesn't exist, than it being a permission issue
This seems a strange error to get when opening an opportunity. One possibility is that you have some java script on the opportunity form that is hard-coded to look for an activity with this Activity Id. Or you may have other customisations that reference
a specific activity which doesn't exist; I'd check any business rules, business process flows and IFrames on the opportunity form / entity
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