GThreadPool crashes with: Failed to set scheduler settings: Operation not permitted

When loading certain pages in my local build of Epiphany, my web process crashes with the following error:

GLib-CRITICAL **: 09:30:04.020: Failed to set scheduler settings: Operation not permitted

(My jhbuild environment uses G_DEBUG=fatal-criticals.)

It seems this is GLib's fault:

(gdb) bt
#0  g_logv
    (log_domain=0x7fe485a460a9 "GLib", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=<optimized out>)
    at ../glib/gmessages.c:1424
#1  0x00007fe4859f4fd3 in g_log (log_domain=<optimized out>, log_level=<optimized out>, format=<optimized out>)
    at ../glib/gmessages.c:1462
#2  0x00007fe485a408bb in linux_pthread_proxy (data=0x7fe418002360) at ../glib/gthread-posix.c:1265
#3  0x00007fe489195f9d in start_thread (arg=<optimized out>) at pthread_create.c:442
#4  0x00007fe4892175d0 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

Looks like this is related to !1356 (merged).