The version of Java which is require by Uppaal is old and known for security issues.
I would like to suggest an update of java version or rewriting the UI, in for instance QT.
Can you be more specific about the version you are using?
The minimum requirement is Java 7, but it can be (much) newer.
I just checked that Uppaal does work fine on El Capitan (10.11.6) with the current Java 8 update 121 as distributed by Oracle.
Which security issues are you referring to?
As for QT re-implementation we need more manpower.
I have installed Java 8 update 121. But when I opened Uppaal it redirected me to 1.6 installation page from Apple. So I installed that.