Problem was while entering/editing links in admin backend and while re-importing a previously exported file.
Weird, I cannot reproduce at this point (in admin backend) and will close the case.
Thanks.
PS, for your reference, my collations are (standard, did not change anything):
| wppfs_linkcategorymeta | meta_id | NULL |
| wppfs_linkcategorymeta | linkcategory_id | NULL |
| wppfs_linkcategorymeta | meta_key | latin1_swedish_ci |
| wppfs_linkcategorymeta | meta_value | latin1_swedish_ci |
| wppfs_links | link_id | NULL |
| wppfs_links | link_url | utf8_general_ci |
| wppfs_links | link_name | utf8_general_ci |
| wppfs_links | link_image | utf8_general_ci |
| wppfs_links | link_target | utf8_general_ci |
| wppfs_links | link_description | utf8_general_ci |
| wppfs_links | link_visible | utf8_general_ci |
| wppfs_links | link_owner | NULL |
| wppfs_links | link_rating | NULL |
| wppfs_links | link_updated | NULL |
| wppfs_links | link_rel | utf8_general_ci |
| wppfs_links | link_notes | utf8_general_ci |
| wppfs_links | link_rss | utf8_general_ci |
| wppfs_links_extrainfo | link_id | NULL |
| wppfs_links_extrainfo | link_second_url | utf8_general_ci |
| wppfs_links_extrainfo | link_telephone | utf8_general_ci |
| wppfs_links_extrainfo | link_email | utf8_general_ci |
| wppfs_links_extrainfo | link_visits | NULL |
| wppfs_links_extrainfo | link_reciprocal | latin1_swedish_ci |
| wppfs_links_extrainfo | link_submitter | latin1_swedish_ci |
| wppfs_links_extrainfo | link_submitter_name | utf8_general_ci |
| wppfs_links_extrainfo | link_submitter_email | latin1_swedish_ci |
| wppfs_links_extrainfo | link_textfield | utf8_general_ci |
| wppfs_links_extrainfo | link_no_follow | latin1_swedish_ci |
| wppfs_links_extrainfo | link_featured | latin1_swedish_ci |
| wppfs_links_extrainfo | link_manual_updated | latin1_swedish_ci |