Thank you very much! I've found and fixed the bug.
1. Open file /administrator/components/com_jcomments/admin.jcomments.migration.php
2. Find code:
$db->setQuery("SHOW TABLE LIKE '%jxcomments_threads%';");
and replace with:
$db->setQuery("SHOW TABLES LIKE '%jxcomments_threads%';");
3. Find code:
$comment->source = $language;
$comment->lang = $source;
and replace with
$comment->source = $source;
$comment->lang = $language;
After this import will work!