Never got an error, actually. It just lagged on the immigration, and now it says
"East Continent - You already have the maximum allowed nobles there."
...Which is quite obviously no longer true, as my advie is already in a new realm in the Colonies.
Perhaps its remnant code not catching up? Something making the game believe a relic of Arhlix stayed behind, thus "Occupying" the adventurer slot until it is somehow wiped?
Or am I just an idiot and some new rule or mechanic means I just screwed myself out of having an adventurer in EC?