Casting to Web interface?

[Donut]

In my setup, i'm using the standard web interface as main user interface on my TV accessing my jellyfin server 10.10.3. My TV is just connected as a monitor to a HTPC running Kubuntu and Firefox to display jellyfin web.
I've now tried to cast a media file onto this web interface using my Android smartphone running Jellyfin for Android 2.6.2. But the web interface client is not selectable in the list:
   
But on the admin interface the device is listed:
   

Is casting to the web interface supported? If yes, how to I do this?

Additional Information:

• When using Kodi as client on the HTPC, the cast works fine
  
• I am kind of stuck to using the web browser because I have a custom client script to add a custom link as a card which is not supported by jellyfin/plugins
  
• All clients use the same user "media"
  
• Also on my Notebook web interface, the other (active) HTPC web interface is not visible in the cast list
  
• No proxy/reverse proxy
  
• Network is a private LAN/Wifi network with the router as the only Wifi access point
  
• All clients are connected to the private network and are not accessing from the internet
  

[TheDreadPirate]

Do you have a reverse proxy setup handling all requests?

Chromium and Firefox both work as casting targets from my testing.

[Donut]

Ah, sorry. I forgot to include the network setup information:
No Proxy/reverse proxy in use
Network is a private Network behind a NAT for internet access
All clients are part of the internal network, i am not trying to access anything from the public internet.

I've just discovered the following thing:
When using my smartphone (using Wifi) I can control the web interface on the notebook (also Wifi), but not the HTPC webinterface (LAN connection).
So i suspect that something is blocked by the router (which is also the only access point) when transporting traffic from Wifi to LAN or vice versa. But I can't imagine why the router should do such things. The Wifi I am connected to is not the guest SSID but the standard SSID which should normally be fully connected. The fact that the jellyfin server itself is found confirms that. The server which is running the jellyfin server is also connected via LAN to the network.

edit: added the network information to the original post

[TheDreadPirate]

Does your router put the WIFI on a separate VLAN? Is AP isolation enabled on your router?

[Donut]

I did some further investigation. It seems like it does not depend on LAN/WLAN like I thought:

1. Removed HTPC from device list on admin panel
2. Re-login with HTPC to server
3. HTPC is in Device list and can be remote-controlled
4. Closed Firefox window on HTPC
5. Device is gone from cast list (as expected)
6. Re-opened Jellyfin web interface on HTPC in firefox
7. Device does not show in cast list
8. Reboot HPTC
9. Device does not show in cast list

There seems to be some kind of time-out. When trying this the next day, the HTPC usually re-appears in the cast device list without re-login.
When doing the same with my notebook, the notebook immediately re-appears on step 7 (also Firefox on Kubuntu).

Is there some logging on the server on the Web clients which I could investigate for differences?