Error Message: URL No Longer Exists

Error Message:
URL No Longer Exists: You have attempted to reach a URL that no longer assists on Salesforce.com (see example screenshot below).

You may see this error message for one of the following reasons:

1. If trying to access a page in Salesforce that no longer exists, check the page again and make sure it is same page or tab you were trying to access originally.
2. Check that you have latest Opero Documents package installed from AppExchange.
3. Check that your document action code is matching what is in the custom button code. If you make an important change to document action, the code will change at bottom so important that you update it.
4. Check Salesforce Sharing Rules under Setup for “Document Action” custom object. Make sure custom object is set to edit/public access so all users can access this object. Cannot be restricted or read only if that user wants to generate documents.
5. Error could be caused by that Salesforce user not having access to all or some of app package components. You can check app components by going to Setup > Installed Packages > Opero Documents > View Components. Make sure user has access to all of them.
6. User trying to access Opero Documents Configuration tab or document action object is not an active or assigned app user.
7. If unable to resolve this error from all issues above, please submit a case to Support for additional assistance.