Okay, a bit odd, but I got things working. I noticed if I took off the /Gizmo/ from the URL, things worked. I could see the Library server page, and get links to Webremote, Gizmo, etc. All of them except for Gizmo worked fine. I ended up copying the "Default" directory to another directory and selecting that as my skins in options and suddenly everything worked. Not sure if it was something wrong with the default install, or what, but it is working now.
Agreed with IOStream... ran into the same issue. In my case, running 19.0.117; tried a un/reinstall and a bunch of mucking about until I finally figured out what the previous poster meant. To add some detail:
1. Browse to your J River install, then Data/Library Server/Gizmo/Default.
2. Copy all these files to your clipboard.
3. Move up one directory level (Data/Library Server/Gizmo) and create a new folder (I called mine "Main"; shouldn't matter).
4. Paste the files you copied in step 2 to the new folder you created
5. In J River Media Center, under Tools > Options > Media Network > WebGizmo, change the Appearance to the folder you created in step 3.
6. Restart your computer. You might be able to get away with just restarting J River's Media Server, but nothing seemed to fix things until I rebooted.
Obviously something's up with the install or app if this workaround is required, but this isn't too painful at least.