Error 70 is permission denied. If your macro is running multiple times in quick succession it is possible that the text file is occasionally slow to close so you are trying to open the file again before it closes and releases the handle.
Can you change the approach to open the text file once, process your folder of files and then close it at the end. This would avoid the potential for the issue above and likely make you macro more efficient. If not you may need to check that the file has closed before the macro finishes.
Found that it also depends on which folder is being processed. I had one folder and no error at all (processed all 250 files) and then another folder gives the error after processing around 40 files. (and the error occurs with the Missing Descriptions.txt file which is unrelated to the folder locations)
Got passed the error by having each txt file open at the beginning of processing all the files and closing after it. Before, it was opening and closing the txt files for every file.
It also runs a bit faster now.