Sounds like the issue is re-appearing from the launch of War Within.
Was mainly an issue back then with danish internet provider Norlys, but since Norlys is busy buying companies like telenor, it’s possible its affecting other ISP’s aswell.
Theres a whole thread dating 6 months back about this issue right here:
Conclusion back then was that Norlys was aware of it, Blizzard was aware of it, but neither wants to take responsability.