Exporting MP3 files as "multiple"

Having recorded a tape with 20 tracks, say, I have done the amplification effec and then divided it into tracks with titles for each.
Then, when I come to “export multiple” it sends the tracks to a predefined file. But after it has successfully sent 5 or 9 tracks it stops and a message appears which states “unable to locate target file”.
The whole Audacity file is from that moment no good. It cannot be sent again. Even if I delete the 9 successfully dispatched tracks it still comes up with the “unable to locate target file” message when I try to export it.
It’s infuriating as it takes a fair time to prepare a tape for exporting.
What going wrong, please ?

You can get that if you put a date in your filename. You can’t use a slashmark.

Upper and lower case letters, numbers, underscore and dash only. No other characters in a filename.

I don’t use spaces, either.

Koz