pgfplots cannot read file

The error message is generated if pgfplots cannot open the input file.

That, in turn, indicates search path problems of some sort: apparently, it did not find the file.

Please ensure that it is on the search path. Is it in the same directory of your .tex file? If not, is it on the path for TEXINPUTS?

I recently encountered this problem with MikTex on Windows 10 and pgfplots 1.14:

It did not open data files without a file extension, so i had to rename my files from "data123" to "data123.txt" and change the path accordingly to make it work. This is probably a pgfplots bug?