Indeed, it does help! No more crash, on Linux as well as on AIX.

separate frame for the ediff control panel. If you *do have* one, have the toolbar for the ediff control panel enabled (i.e. ediff-use-toolbar-p set to t) and press the "Toggle split mode between side-to-side and one-on-top-of-another" button, all toolbar buttons except for the "A" and "B" buttons and the "filled space" between the "Help" and the "Quit" button disappear. (No matter whether the control panel is "small" or "large", i.e. This is 21.2.31, but I see it also with 21.1. Am I the only one having this weird behaviour or can anyone reproduce this?