A recent security vulnerability on Java prompted Oracle to release critical patches for the JDK in versions 6u101, 7u85 and 8u51.
This upgrade prevented OMERO clients from connecting to the server. A fix for this issue is now available, you should upgrade your Java version to take advantage of the security fix and refer your system administrator to
https://blog.openmicroscopy.org/tech-issues/2015/07/21/java-issue/
for details of how to patch your server. You do not need to download new clients and this fix will also be included in the next release of OMERO (5.1.3).
— July 22, 2015