Many Packages quit with Segmentation Fault after Minor Upgrade (11.0 to 11.1)


I'm not sure if this is related to the base system or some specific port.

I'm running FreeBSD as a desktop machine on a ThinkPad X220.
Ports/packages are managed by ports-mgmt/synth and packages are fetched from url: "pkg+${ABI}/latest".
After upgrading from FreeBSD 11.0-RELEASE-p1 amd64 to 11.1-RELEASE-p4 as described in (or as I understood) the handbook via
# freebsd-update -r 11.1-RELEASE upgrade
# freebsd-update install
# reboot
# freebsd-update install
and doing
# portsnap fetch update
# synth update-system
many applications crash sooner or later with a segmentation fault.
It seems that the duration until crash is somewhat related to resource intensity as www/firefox or mail/thunderbird segfault nearly immediately while www/dillo or mail/claws-mail may run for an hour (depending on usage).
This didn't happen to X window manager itself (I'm using x11-wm/xfce4 and x11-wm/i3) or command line programs (I tried www/links, net-im/profanity, and others), but I cannot say if they would crash eventually too if running long enough.

I enabled coredumps via sysctl kern.coredump=1 to see if I could narrow the problem down to a single library or function call. Unfortunately they weren't very helpful to me, for example:
(gdb) core thunderbird.core
Core was generated by `thunderbird'.
Program terminated with signal 11, Segmentation fault.
#0  0x000000080207b84a in ?? ()
(gdb) core claws-mail.core
Core was generated by `claws-mail'.
Program terminated with signal 11, Segmentation fault.
#0  0x000000080b36dba9 in ?? ()
I thought it was unnecessary to recompile all ports when doing a minor upgrade.
Nevertheless I did a synth force `pkg query %o`, which took quite long but didn't help.

Finally, I gave up and made a (dirty) rollback via
# mv /boot/kernel /boot/kernel.11.1
# mv /boot/kernel.old1 /boot/kernel
Everything is running as stable as before, now.
What else can I do to isolate my problem? I tried ktrace and kdump, but I've to admit that I don't understand its output as it was full of error messages which may have nothing to do with the described issue.

Thanks in advance.
Besides, there are library changes between 11.0 and 11.1. I only noticed that when trying to run 11.1-built postgresql on a machine still running 11.0 (last update). So I rebuilt them all, just in case.
Thanks for your hints which led me in the right direction (I think). It's all my fault:
When setting up my system (11.0) some time ago, I chose Prefetch prebuilt packages in synth configure (which isn't recommended anyway, if I remember correctly) to accelerate the initial installation, and then forgot about that.
Now, I see a lot package files in /var/synth/live_packages/All which predate my upgrade from 11.0 to 11.1. So, my local repository seems to be a mess and I've to do some cleansing now.