You sure that you run that all as root?
tried to reboot the slug?
Did you run ipkg update / ipkg upgrade?
Tried to run ipkg with more verbosive error messages?
I guess you lack some deps, or some deps are not met with the proper version.
Strangely enough though, I never had any problems at all running ipkg install openssh on a fresh unsling slug.
Yes Yes and Yes
The only thing that I gleaned from the verbose message is that the files contained no MD5SUms. I even tried installing an older version.