In my current Minecraft World on my Minecraft Realms i've found a Ruined Portal which i repaired and lit up.
After entering the Portal i got the usual
"Generating World
 Building Terrain"

Screen, but while being stuck in that Screen i heard myself getting damage, just to find myself dead "suffocated in a wall" while there was no wall.
Either the game thought i was stuck in the "Nether Portal (Block)" itself, the Obsidian of the portal or just anything.