I have the same problem. Started tonight. v0.69.1 Everything else is working (SAMBA, SSH, other add-ons etc…)
I started getting the 10 seconds warning once my config grew apreciably. I don’t think it has anything to do with this.
I have tried the following without success:
CTRL+F5 browser cache refresh while on the Hass.io (blank) tab. I had to do this every time I visited the tab to get it to display (due to SSL problem with the cert not matching local url - I think) but tonight it makes no difference.
Added hassio: to customization.yaml
Restart.
Stop server and reboot pi.
Was just considering burning a new SD card. Your post does not fill me with confidence @Jag.
EDIT/
I have an interesting update.
I installed a fresh copy of Hassio on a band new micoSD card and without configuring anything the Hass.io tab is not visible in firefox or ms edge (both with freshly cleared caches and histories). Chrome however shows the Hass.io tab perfectly.
I then enabled the SAMBA add-on and copied across my latest snapshot and restored from it. Same result. I can see the Hassio tab in Chrome but no other browser or in the ios app.
I then went to my configuration.yaml file and changed
frontend:
javascript_version: auto
to
frontend:
javascript_version: latest
Restarted and… no change.
At the moment Chrome web browser is the only one that renders the hassio tab correctly.