OK, thanks, it works that way.
Two things: Photoshop does not 'save as' jpeg if the file does not support jpeg, for example because it has layers or is 16 bits color. You can restore the old behavior in the preferences under file handling. Check 'use legacy save as'. Or indeed use Save a copy.
I'm a bit puzzled about your second problem. First you say you can't save the file, and then you say that if you saved it anyway, you can't open it. Don't tell me you save it in another format and then change the extension...
Thanks for your answer.
Yes, if I use save a copy, it works : I can have access to JPEG format. Good solution.
The second problem is solved through the solution of the first one. The problem I had, was that I couldn't open the photos that I had saved under jpf format on my desktop.
I have the same problem when trying to save images I opened from Camera Raw. I did change the Color space from 16 Bits/Channel to 8 Bits/Channel, after this I got the option to save images as jpeg.