Thank you! However, I solved the problem by changing the parameters of the qiime import script with this
qiime tools import --type 'SampleData[SequencesWithQuality]' --input-path manifest.txt --output-path /home/genomica/DATA/metagenomica/Bactrocera/sequenze/single_end2.qza --input-format SingleEndFastqManifestPhred33V2
It looks as if the tools import script has changed since this (December 2018) has been posted, and particularly, the old topic mentioned a wrong input-format and wanted a directory with --input-path, not a manifest.txt file. So, it possibly generated a conceptually wrong file... as the validate script detected!
Then, I ran:
qiime tools validate single_end.qza
Result single_end2.qza appears to be valid at level=max.
And everything worked fine! I was also able to continue my analysis with dada2 denoise-pyro with no errors and coherent output!
However, thank you for your support!