I downloaded one of my old mods to remember how I did a certain script, and I cannot view the scripts in the creation kit. "Edit Source" is greyed out, and an error message appears if I click it anyway. Why would that happen?

Your script source file isn't loose in the Data/Scripts/Source folder.

Most likely the source file is not in scripts\source folder, and if you have packed it in a BSA CK won't see it even if you have the BSA path in the .ini file.

Damn Ninja's.......