COS = cache object script, a programming language
Like in pretty much all programming languages, an expression is something that can be evaluated/resolved to some value.
http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=...
Use the Audit log features from the management portal. The error details will tell you more specifically why it failed.
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maybe like this http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=...