I wrote that I had experienced a panic at boot in El Capitan, and that I was hanging off reinstalling some software until I had found the problem.

I found the problem.

I had cause to reboot the system, and it hung again at the boot screen

So, I rebooted in Single User mode (Command-S) and this time I looked closely at the screen.

Enter com_eltima_async_Navel::start(this=<ptr>,provider=<ptr>).
panic(cpu 0 caller 0xffffff80017d6a9a): Kernel trap at …
…

Debugger called: <panic>
Backtrace (CPU 0), Frame : Return Address
…
     Kernel Extensions in backtrace:
        com.eltima.SyncMate.kext(0.2.5b15)…
       

A little research pointed to Eltima’s SyncMate, which I had installed to sync an Android phone. I had removed it some time ago, but it had left behind a kernel extension, which hangs El Capitan at boot (sometimes. But once it’s started, it doesn’t seem to stop)..

I completely removed the vestiges of SyncMate, by removing the files/folders:

• /System/Library/Extensions/EltimaAsync.kext
• /Library/Application Support/EltimaSyncMate/

and all is now fixed.