The relative URI
knime://knime.workflow/…/data/Stores+data_upload_II.csv
does not work, but
knime://knime.workflow/…/data/Stores_data_upload_II.csv
is OK.
KNIME 4.1.1
Windows 64bit
org.eclipse.equinox.launcher.win32.win32.x86_64_1.1.551.v20171108-1834\eclipse_1630.dll
org.eclipse.equinox.launcher_1.4.0.v20161219-1356.jar
Plus sign ("+") is reserved URL character so try %2B instead of it. In case this part needs to be automated you can use urlEncode() function from String Manipulation node.
Anyways there is ticket (internal reference: AP-13670) to have proper encoding in these cases.
String Manipulation is usable, but I used the CSV Reader node, where I can browse a filename and after that I wanted to make the URL workflow-relative.