I have no idea how this started or why this started, but every single character on my account has the following:
And when I attempt to acquire the items, they don’t actually exist:
A full UI reset hasn’t fixed it. It wants me to pick up a Vibrant Shard and a piece of Wildercloth on every single character that actually doesn’t exist. o.O
Oh good, I’m not alone in this!
Okay, that makes me feel better. I didn’t know how I managed to break the game. 
That is usually my job!
This is happening to all my alts as well!
As much as I hate the “me too!!!” threads, I too am so glad I’m not losing it. I had thought perhaps there was something borking up with my alt add-on (I’ve been struggling with it these past few days) and where I sat down and did a good push into the new content, that maybe something had gotten hung up that I hadn’t looted properly.
Off to Bug Reports I go!
I was just about to ask about this, because I let my sub lapse yesterday and today all my toons got this mail. At least it’s good to know it’s nothing on my end to worry about.
It happened on my EU account also.
I thought I was going crazy. Every one of my characters that I’ve logged into so far have the exact same issue.
Happened to me too, on every character I log into for the first time since yesterday. I was generally able to loot one of the two mails, but after looting the first, the second mail’s item would be “not found” and disappear.
I am able to delete the mails now. I just need to attempt to loot the item and be told it isn’t found, then the mails can be deleted.
Now to do this throughout my entire roster of alts, sigh.
Fingers crossed they don’t come back.
This is also making it so that “Trial” or accounts with subscriptions lapsed, can not delete characters.
This character has mail and can not be deleted.
Very frustrating. Please fix and clear these items or allow Trial accounts to access mail for a short period to delete these horrible unlootable and for trial/expired – invisible! spam.
All of my characters have it. And it’s preventing me from deleting a character.
Good times.
You can delete the mail. Open up each mail, try to take the item, it’ll give you an error. The Wildercloth you can take, the shard you can’t. When you close out of the mailbox entirely and re-open it, there are no attachments any longer and you can delete the mail.
Unless you’re talking about a character in Exile’s Reach. Then yes, that does pose a problem. You’ll either have to progress through Exile’s Reach or you may have to put in a ticket.
It looks like a lot of folks received the same thing. I did last night, but I haven’t checked my other characters yet.
I haven’t seen any other reports on it, but I’ve forwarded it up.
It’s on all of my characters, I came here when I realized some of the characters have never in the history of ever possessed that shard as an item. 
My characters on 10.1 PTR also got this same mail
My human monk was one as well.
Have the same problem to thought it might have been a hacker attempt or somthing akin