No airlines have approval to operate in high density ash. Nearly all airlines have demonstrated a case for safely flying through medium density. Unfortunately they believe the high density came over NCL today.
As has been said, the ash is posing significant problems below 20000ft, above that, in alot of areas its fine, hence why planes have been seen flying above NCL.
Yes its causing problems, but at least there are no blanket bans like last year, and the decisions are being made based upon better info.