I've been looking at this a bit, and it looks like this might be due to the newline at the end of the German version of the event. The
end Leo command looks like it's what sets the mail flag and triggers the overnight event, but the German version is
end Leo\n" . I edited my version to remove it and it worked fine after that, so unless it's random (I didn't test it many times) that's probably why, hehe.
It looks like if you skip the cutscene the treehouse should get built either way, though. If you're not using any mods, we could get you past it with a
save edit, either by adding
<string>leoMoved</string> to the
<mailReceived> section (which should build the treehouse without showing it being built overnight), or by removing
<int>6497428</int> from your
<eventsSeen> list so you can trigger it again, then skip it. Make sure you back up your saves first if you want to try!
If you don't feel comfortable with it or it doesn't work, you can zip the whole folder for that save and upload it here.