rejection of wav-files as "not being wav-files"

VERY MUCH Worried! I’ve done that before: To record cassettes (with my old music) to Audacity, then to export those items (one by one or as multiple export) as WAV and as MP3. Afterwards I have burnt CD:s with that music.
Now (a month later) the program rejects my intention of burning them, telling that “This is not a valid or supported music file”. The files are wav-files and I can play them with Media Player. Curiously (!) same files exported as MP3 are accepted for burning.
I have quite many recordings intended to be burnt into CD, and the preoccupation is obvious. Isn’t it? Thanks for any help!

This is not a “forum issue” - this section of the forum is for asking or discussing things about the forum, such as do you like the colour scheme, how to post images…
There are sections for Windows/Mac/Linux which tells us what platform you are on, and there are sections for Audacity 1.2.x and 1.3.x which tells us what version of Audacity you are using.

What operating system are you using?
What version of Audacity are you using?
How do you know that they are WAV files? (some programs such as iTunes and Windows Media Player have a bit of a habit of changing the format of audio files to their own format).