Seems that marking as read some of messages helped.  

If somebody will have with issue too.  Try to enter Group and specify
"How many articles from GROUP read (NUMBER)" a lower NUMBER.  Then you
could mark authors as read hitting <L a e i ENTER> or kill threads with
C-M-k.  I successfully entered my INBOX group again with about 10403
unread articles after marking as read about 500 messages.

Oleg.