Can JRiver and norton live together?
Yes, MC and Norton Security can coexist just fine. I use exactly the same version as you and only occasionally have issues with Norton, usually after an update, or a Windows update.
Jim's suggestion to uninstall Norton is just to test if the problem goes away. Also, if it does go away, when you reinstall Norton it will sometimes detect existing programs and correct any bad settings for them.
If the problem goes away, reinstall Norton and see if it stays away. If not, tell Norton not to interfere with MC, particularly the Firewall, but also the Anti-virus.
Make sure Norton is not in Silent Mode, as you need to see messages it generates and take proper action, such as allowing MC components to download or run when required.
Of course, it could also be that some of your m4a files are triggering the Anti-virus. Shouldn't happen, but could be. Scan the files with Norton and confirm no messages.