It looks like the JavaAction ‘GenerateExcelDoc’ is unable to find the document. Have you tried re-uploading the document? You could also try to delete you deployment folder in the Mendix app if it happens locally. Beware: by deleting the deployment app you will lose all you data, so also useraccounts and the like.
how many records you were downloading into excel? i had similar issue before, after analyze in details, i found it caused by heap out of memory. so i did some enhancement for the excel export module.