Hi there! Please try the following steps. Those have solved this very same issue all the time:
1. logging out
2. opening the daoc folder and starting "camelot.exe"
3. waiting for the patcher to fully patch the game.
4. downloading the atlas pack ( https://www.mediafire.com/file/bqoxe9xh89lmg2d/AtlasPack-0.3.zip/file ), copying it into the folder and unpacking it.
5. allowing replacements, if necessary
6. starting atlas launcher as admin
I just had this same exact problem. I had no issues testing Atlas, then I updated by running the camelot.exe in my Atlas game folder, then tried to play on Atlas as I had been doing, and I could not log into Thid. I had to download the Atlas pack again and extract that to my Atlas game folder.
I guess this happens because the Atlas launcher doesn't do any file checking or installing of files?