Impossible to convert char to date

[so10070]

I try to convert a character field to a date field.  The content of the field ([The 1st of September of the current schoolyear]) is

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cast ([Date schoolyear] as varchar(4)) + '-09-01'
The [Date schoolyear]-field is a standard field of the BI.
Then I try to put it in a filter

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[Earliest date assignment] >= cast([The 1st of September of the current schoolyear] as date)
I get always a date/char error!
In orther report it works fine! :(

[dougp]

What is the actual error message?
Can you provide at least one sample value from [Date schoolyear]?
Are there any values in [Date schoolyear] that do not look like 4-digit integers?

[so10070]

Problem solved.  Was an error of Cognos itself! (dataconnection and so on)  :)