Comment # 3 on bug 1153847 from Neil Rickert

Unfortunately, there appears to be newer hardware that also doesn't support
multi-signed shim.  We see that in forum posts.

Check https://en.opensuse.org/openSUSE:UEFI
and scroll down to the heading 
"Booting the Machine that supports only one signature with vendor provided
Keys"

It suggests a workaround.  And you can use that workaround as a way of testing
whether that is the problem.