Export to MP4 produces (not readable) .M4V- and .ACC-file

Hello,

I use Premiere Elements 15 and  Windows 10.

When I finished my last project, I exported it to MP4. Everything was ok.

Now I added  text and more music (.WAV) and exported again.

The result is not a MP4-file. Instead I get a M4V-file and a ACC-file.

The size is the same (5.61GB). If I want to view the file, I get an error message (wrong format,....).

What happened, that  I don't can export to MP4 as before?