incompatible Qt library starting LyX

[Will Parsons]

My installation of LyX has stopped working, with a message:

Cannot mix incompatible Qt library (version 0x40806) with this library (version 0x40807)
Abort (core dumped)

I had updated some ports a couple of days ago, but nothing that I
thought would have affected LyX.  I reinstalled lyx, qt4-corelib, and
qt4-gui, but with the same result.  I don't see anything in UPDATING
that looks relevant.  Does anyone have any idea how to fix this?

% uname -a
FreeBSD anukis.local 9.3-RELEASE-p21 FreeBSD 9.3-RELEASE-p21 #0: Tue Jul 28 08:57:41 UTC 2015     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  i386