It may be data, but what it could really be, is a corrupted SD Card. If it is not letting you install .cias than that could mean some sort of a write error. Copy all data off of the card while you still can, and just get a new SD Card from Amazon, 128 GB cards only cost around $20, I suggest Samsung, or Sandisk, keep an eye out for fake knock off cards as well, because they plague the site.
Also this same error occurs for me whenever I try to install a .CIA from a random site on the web. Look for them on Github, or by using the homebrew browser Universal Updater.