In case it is not obvious, I can launch custom client but it shows nothing but the name bar (not sure of the correct term.) How can I fix it? Trying to edit it run.bat according the instructions I found on the forum doesn't help it just completely fails to launch.
EDIT: I managed to add default client into Java security exception and launching it only for it to bug out similarily as my screenshot of Enders. Java.com claims I have Version 8 Update 25 installed.