This caught me up, thought I would post here about it.
I was getting this error when I clicked on a file to edit it:
The file could not be found in {kirby_url}/panel/app/controllers/views/files.php on Line 135
Turns out it was because the filename had an ampersand & character in it. When I removed the ampersand character from the filename, everything worked fine. I had placed the file in the content directory manually. If I had uploaded it through Panel, Kirby would have replaced the & with a - automatically.