Without Easy Open installed, whenever you double-click on a file, your Macintosh tries to find an application with a matching "signature". If there is one, the application is opened and the file is blindly given to that application to be opened. If there is no matching signature, a message appears telling you the application could not be found (sometimes with an additional option to open the file using TeachText).
With Easy Open installed, every time you double-click on a file, Easy Open searches certain resources located inside your applications with a matching "signature" as well as Easy Open's translation components to ensure that the application can open that file.
Due to the sequence in which Easy Open searches, the combination of any version of Easy Open with Excel 5.0 and MacLinkPlus for Easy Open 7.5 (which is aware of Excel 4.0 but not Excel 5.0) causes Easy Open to not allow Excel 5.0 open its own document. This is not a bug in any of these individual components, but a problem with how these components interact when they are not aware of each other.
Workarounds
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Short Term Solution
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Turn Macintosh Easy Open off. Macintosh Easy Open is located in the Control Panels folder.
Long Term Solution
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Use MacLinkPlus for Easy Open 7.52p1 (RD-MLP-EASYOPEN). This can be obtained by contacting DataViz technical support.
...or...
Use MacLinkPlus version 8.00 or higher.
...or....
Upgrade to Microsoft Excel 5.0a which fixed the problem.
The Tech Info Library article titled "Locating Vendor Information" can help you search for DataViz Inc.'s address and phone number.