Further information: It may be that at least some of the problem lies with Virtual DJ, not WavePad, because when I tried to play the same file on Groove Music media player, I got a message that the file was corrupted and could not be played. So I don't want to waste anyone's time trying to solve a problem that may have been created by another app. However, that does not explain the first error message I got on WavePad. Investigation continues, and suggestions still welcome.
I am having a similar problem on Windows. I create audio mp3 files for radio on a program called Virtual DJ. I then edit them on Wavepad. I have been doing this for about a year, consistently, with no problems. Then all of a sudden today I got error messages as I tried to open a particular file. First I got a window with a red X error message on it saying that it could not open the file. After several failed attempts with the same result, I restarted my computer and I was able to open the file. But when I tried to open another file, I got another error message -- this time with no red window b