mergemaster failing with read-only /usr/src

Hi,

I'm trying to update this system:
FreeBSD pomona 11.0-CURRENT FreeBSD 11.0-CURRENT #0: Mon Apr 13 03:48:04 CEST 2015     wolfgang_at_pomona:/usr/obj/usr/src/sys/UBQTERL  mips

Source for that was probably from about April 11th. I sucessfully built
world and kernel, ran mergemaster -p and make installworld on rev 282299
but then mergemaster fails with:

# mergemaster -iFU

*** Creating the temporary root environment in /var/tmp/temproot
 *** /var/tmp/temproot ready for use
 *** Creating and populating directory structure in /var/tmp/temproot

/bin/sh: cannot create routing_test.tmp: Read-only file system

  *** FATAL ERROR: Cannot 'cd' to /usr/src and install files to
      the temproot environment

Filesystems are mounted like this:
# mount
/dev/da0s2a on / (ufs, local, noatime)
devfs on /dev (devfs, local, multilabel)
/dev/da0s1 on /boot (msdosfs, local)
vulcan.lyx:/usr/src11 on /usr/src (nfs, read-only)
vulcan.lyx:/var/obj/11/mips64 on /usr/obj (nfs)

This used to work before. Any ideas, any further info I could provide?

Wolfgang