While trying to administer some Net App's I would get his unhelpful error message trying to log in. I assumed it was a Java deal, and tired uninstalling/re-installing, various flavors, all w/o help. The newest On Command System Manager 3.1.2RC2 is supposed to work with Java 8.x. Oddly I could log onto the 3250's but not the 3250's. All are running v8.2 7-Mode.
Thanks to Cody's Blog for posting up the answer. Turns out some Net Apps still use SSLv3, which is disabled by modern browsers, turning TLS on (on both controllers) allows one to log on!