Getting "cannot locate default form" error

I have a user (using R5) who gets the following error message when he tries to forward an e-mail: "Can not locate default form." I have replaced/refreshed the design, removed the cache.dsk and we're still having this problem. Any idea how to solve this?
When you press the Forward action, Notes looks in your current location document to find the location of your current mail file. Notes then looks for the default form in this file, which is usually Memo.

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I suspect that the mail file is not specified correctly in your location document. I would start by checking that.