Iced Tea is the version of Java that ships with most Linux distributions, but Oracle also makes their official version of Java available for download. If both the official version and Iced Tea are installed at the same time, such as when you are running Iced Tea for Java 7 and then upgrade to Java 8 which is only available in the official version, Ignition Designers and Clients can experience freezes under some circumstances.
For Clients, the easiest workaround is to launch using the Native Client Launcher available on the Gateway's Home page.
For Designers, you will want to make sure that java and javaws both point to the same version of java. You can locate java with the command
$ which java and locate javaws with the command
$ which javaws. Both of this will probably show you the location of simlinks to the actual files. Follow the simlinks to find which version of java or javaws they point to. (For example, if you follow the simlink at
$ which java, you may end up at
/usr/lib/jvm/java-8-oracle/jre/bin/java, which tells you that you are using the official Oracle version of Java 8.
If java and javaws are pointing to different versions of java, there are several solutions. One is to launch the designer jnlp file from the command line, using the full path to the javaws version you want to use. Another solution is to remove all versions of Java and then reinstall only the version you want to use.