Without more details or being able to look at the actual trouble jars I can only guess.
A lot of people are having trouble with 1.7's update 45 with web start. Some of the things causing trouble include:
JNLP-INF/APPLICATION-TEMPLATE signingUsing the versioning protocol breaks attributes
Check the release notes to see if you're running into known issues:
http://www.oracle.com/technetwork/java/javase/7u45-relnotes-2016950.html
I've seen this error for a different reason. I build a project that has a common library I try to reference as an extension resource. Unfortunately my build process also pulls the same library in under lib/ and references it in the main jar's manifest. The library jar is built and signed twice and for some reason is slightly different in size. This trips up the javaws loader with errors about signatures. That was hard to sort out because I would look at the extension jar and
test it's signature and it was fine, yet it's name was appearing as the one being unsigned.