The Save report action in the Web Viewer UI does not encode the file name correctly when the "Save as" field contains invalid url characters.
Om Web Viewer 14.0
Here are some that I found that causes problems:
Plus sign (+) - Converted to space
Pipe (|) - Bad Request
Backslash (\) - Bad Request
Curly braces ({ and }) - Bad Request
Square brackets ([ and ]) - Bad Request
Caret (^) - Bad Request
Percent (%) - ReportData
Hash/Pound sign (#) – ReportData
CIRCUMVENTION:
Save the report with a file name containing only valid url characters.
For example, by making sure the "Save as" field contains only alphanumeric characters. The saved file can be renamed to include the invalid url characters as long as they are permitted by the local operating system.