I figured out more details concerning my problems with long descriptions: old texts are still there, but since the last update the plug-in does not write in wp_links_extrainfo. There is no data set for new links. If I create an entry in wp_links_extrainfo it works fine. I can change the long description and it appears on the listing page, but for new links it does not create an entry. Maybe that is helpful for you. Now I will find out, why it does not write to wp_links_extrainfo with first submission of a link.
Maybe I get a solution on link library, before I solve it myself.