It sounds like you’ll need to update the Java Virtual Machine (JVM) version that OpenWebStart is using. It has a built in JVM manager for doing this. Personally, I got tired of messing around with OpenWebStart and the confusing Java version situation after v8 (caused by the Oracle licensing mess). Now I just use the JOSM installers that come with their own bundled Java version so I don’t have to think about it. I miss automatic updates, but they don’t come so often that it’s a huge issue.