As soon as you reconnect the cabel and get internet the blank launcher appears again. So the problem is really in the account login part.
You could play in offlinemode with a mod playset, which you enabled before the bug appeared.
I managed to induce my own fix, I'm not sure of what part was the fix but my steps:
1. I VPN'ed to Sweden since last time i had a bug like this was with a Japanese game and a Japanese vpn fixed it...
2. exit and reenter steam.. steam was confused for a bit, then loaded.. i tried to launch- it still failed
3. set network back to no VPN (so my home Aussie internets.)
4. Pressed play again on stellaris, noticed the buttons existed for long enough to press, pressed play without thinking
5. stellaris launched. YAY. closed stellaris to retest
6. now launcher is loading properly??
so either the vpn flip fixed, as during step 3/4 when steam loaded ALL paradox games flickered "validating" (no other game did?) on step 4 i didnt confirm the fix, because i rushed the "press button before it disappears!" which also may have been the fix?