Thank you! Now I succeeded in installing Horde 5.2.
Unfortunately, many things don't work. I can login and check my mail, that's fine. But I get database errors, whenever I try to explore the new version. If I, e.g., select Calendar, I get:
HORDE: QUERY FAILED: Unknown column 'share_parents' in 'field list' INSERT INTO kronolith_shares (share_name, share_parents, share_owner, attribute_name, attribute_color, perm_creator, perm_default, perm_guest, share_flags) VALUES ('thN6FmM7fpmc01of3B_ZIVt', NULL, 'holck@dk', 'Calendar of holck', '#a3dd66', 0, 0, 0, 0) [pid 10941 on line 1669 of "/usr/share/pear/Horde/Registry.php"]
I guess I need to do some database-related update?