No disc issue opening epubs

:help:I have recently installed the latest version of Calibre and I loaded some e-book, that are .pdf and .epub only.
Where with .pdf files Calibre works perfect, it doesn't with epub ones.
Whenever I try to open an epub file I get an error message about:
"Calibre-Parallel.exe - no disc" :angry:
What did I do wrong?
Gbegno.