upgrade D6 => D7 and menus with "umlaut" (ä, ü, ö) and other special characters (™, ®) gives unserialize() error

i just needed 2 days to migrate a D6 website with german "umlaut" (ä, ö, ü) and other special characters (™, ®) in the menu names and description to D7. the problem is that after the migration the following error is shown multiple times on every page.

•Notice: unserialize() [function.unserialize]: Error at offset 54 of 57 bytes in _menu_link_translate() (Zeile 869 in /var/www/html/includes/menu.inc).

additional it is not possible to edit the menu with such links after the migration. ther following error is shown on menu, list links:

•Recoverable fatal error: Argument 3 passed to l() must be an array, boolean given, called in /var/www/html/modules/menu/menu.admin.inc on line 102 and defined in l() (Zeile 2339 von /var/www/html/includes/common.inc).

the only way i got it running was to modify all menu links and desctiptions on D6, then migrate to D7.

not sure from where it comes and if it has to do with this problem - the mysql db from the hoster i use is utf8_general_ci, but some tables (also the drupal_menu_links) is latin1_swedish_ci.

the error can be reproduced very easy.