Do you mean that the EPUB is encrypted, or do you mean that the EMBEDDED FONTS in the EPUB are encrypted?
I know that no EPUB I've created from InDesign CC is encrypted because I can open it up in any EPUB reader. It may be that embedded fonts are encrypted. I've never experimented with embedding fonts because of the complications between different EPUB readers that I've not had time to sort out. Try not embedding fonts by unchecking Include Embeddable Fonts on the Advanced tab of EPUB Export Options.
The error message says that the epub itself is encrypted. I haven't had this issue before. I can open the file up in Sigil and other programs (including DW), so I've been mystified by this error message. However I just took your advice and unchecked Include Embedded Fonts and it worked!!