I found an explanation how it should work.
Deleting the last_activity_id field with this SQL command
SELECT * FROM
options where option_name = 'g_crossposting_last_activity_id'
It worked once. But now I would like to do it again to import some more previous posts.
I deleted all old posts and deleted the id field again.
But no luck this time. Somhow I cannot re-import.