Hello Robyer,
many thanks for your reply! And yes, you're right, it is the plugin Message Export. I updated the thread title and the original post.
If bug reporting wouldn't take so much time, i would love to do it more often. And, frankly, my contacts don't care much about IM anymore, which is a pity. So, i write and receive only some few messages a week.
Wish you a nice weekend,
Ravelli