The URL must be encoded as described here https://www.flexibee.eu/api/dokumentace/ref/filters
"Filters must be properly URL-encoded. When creating a filter manually, you can simply type it unencoded in the Firefox browser. When you then copy it to the clipboard, Firefox will re-encode it."
The command should therefore look like this, without spaces (or encode them as "%20"):
curl -u winstrom:winstrom -k -L -f "https://demo.flexibee.eu/c/demo/faktura-vydana/(datSplat%3Enow()).xml" -o ds.xml
Encoding reference https://www.w3schools.com/tags/ref_urlencode.asp