My Characters Lost Their Names And Can't Log Them In

After a couple of months of inactivity (mother was ill, and then died) I came back to game only to see that some of my characters lost their names and when I try to log them in I get disconnected from the server.
Out of ten of my characters six lost their names, and four of them are level 90. The other two are 64 and 63,
How that error can be corrected?
Yes, I downloaded the fresh client.
No, I checked for viruses and none found.
Yes, I keep my Windows and drivers up to date. (Windows 10, 64 bit)
Yes, the remaining 4 characters that still have their names CAN be played.
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EDIT: After several attempts more and more of those toons got the option to rename, so I did it.
What happened to my old names, were they stolen?

After few month of inactivity your name, including mine get released. Since your server is dead enough prolly no one steal your name and you can use your old name.

First and foremost my condolences to you.

As kairxa mentioned you can attempt to retrieve the names of your characters just by renaming them. If not possible then you'll need to pick different names.