Hi @solita_lol I'm sorry to hear that you are experiencing issues starting the Dawn of Ragnarok expansion.
I can confirm that the issue with the image not loading under add-ons is a known issue, however this should not prevent you from accessing the expansion. You did however mention that it is showing as owned and not installed, this would mean that the expansion has not been installed which would explain why you are unable to access it.
Can you please go into Ubisoft Connect > Click on the Games tab > Click on Assassin's Creed Valhalla > Scroll down to Owned DLC > Click to install Dawn of Ragnarok if it's not showing as installed.
Once installed try launching the game again and see if you are able to start the Dawn of Ragnaro...