I am seeing the same thing.
NGG+ was working fine on my site. Upgraded to 4.2.1; still worked fine. Then I tried to change a theme, and that’s when trouble started: wp-admin page went blank; with debug enabled I get different errors when page is reloaded, all relate to memory allocation overflow, citing different PHP files.
On another site I did a new install of WP 4.2.1 and installed NGG. It immediately turned wp-admin screen blank. I renamed the nextgen plugin folder and regained WP admin screen; disabled and enabled NGG and it started working. Added NGG+ and it is still working. However, LR/Blog plugin is unable to connect to NGG on this site, although if I switch LR/Blog to WordPress mode it authenticates the user name and password successfully.
Other plugins were all disabled and removed, and re-installed one by one and are the same on both sites now.
The site that was upgraded to 4.2.1 still can not enable NGG.
Something is broken.