It seems it's a vanilla Solaufein bug, then, judging by the fact that you've got nothing else Solaufein-related installed. Either use CTRL-Q cheat at your own risk(should work, though), or well, don't drop him from the party.
I've got little problem. I dropped Solaufein from my party at the top of Watcher's Keep - and now that I am back to pick him up again he does not join the party.
This is what Solaufein says when I talk to him:
"I have nothing to say right now, my friend." (PC's response)"Very well...let's go."
What is this about? Is this a bug or am I missing something?