Hi, The 10.4.3 update to OS X breaks a significant amount of Chimera functionality. Attempts to use that functionality cause Chimera to exit. In particular, for structures that contain residues other than the standard amino/nucleic acids or water, anything that directly or indirectly makes Chimera determine atom types will cause the failure. Such things include adding hydrogens, finding hydrogen bonds, and going into ball-and-stick or sphere representation (the atom sizes are based on atom types) -- though stick-only mode is okay.
We are working on a fix. Until we have one, you will have to weigh the improvements offered by 10.4.3 against the loss of Chimera functionality. I'll send mail to the list as soon as there are developments.
--Eric