Holly cow!!! Forget my last message, I finally found the problem!
For some reason, there was a mistake in the pile of certificates in the generated pure-ftpd.pem
After the first or second certificate, a line break was missing, which was creating a problem to read the rest of certificates insite the file, obviously :-D
-----END CERTIFICATE----------BEGIN CERTIFICATE-----
I guess it,s something to let users know about. I hope my fixing will help others!
Anyway, a big big thank you Till, you saved my life!
Cheers and hail to ISPConfig ;-)