2.1.5 to 2.2 update failed (Unable to open /cf/conf/config.xml for writing..)

[jstrebel]

Hi,
I tried to do a manual upgrade on my PC engines APU Board, which boots from the SD Card. The upgrade fails and and produces the following error message:
[ Unable to open /cf/conf/config.xml for writing in write_config()]

The Box comes up again with the version 2.1.5 and runs nicely. I was able to download the config.XML file. The file itself looks ok it is not corrupted)

Below are some details which may eventually help.

Any suggestions what should I do next.

Thanks in advance Jakob

$ df -h
Filesystem          Size    Used  Avail Capacity  Mounted on
/dev/ufs/pfsense1    1.8G    202M    1.5G    12%    /
devfs                1.0k    1.0k      0B  100%    /dev
/dev/md0              38M    2.1M    33M    6%    /tmp
/dev/md1              57M    18M    34M    34%    /var
/dev/ufs/cf          49M    1.8M    43M    4%    /cf
devfs                1.0k    1.0k      0B  100%    /var/dhcpd/dev

/sbin/mount
/dev/ufs/pfsense1 on / (ufs, local, noatime, synchronous)
devfs on /dev (devfs, local)
/dev/md0 on /tmp (ufs, local)
/dev/md1 on /var (ufs, local)
/dev/ufs/cf on /cf (ufs, local, noatime, read-only, synchronous)
devfs on /var/dhcpd/dev (devfs, local)

Jan 25 09:39:39 php: /diag_nanobsd.php: New alert found: Unable to open /cf/conf/config.xml for writing in write_config()
Jan 25 09:39:39 php: /diag_nanobsd.php: WARNING: Config contents could not be save. Could not open file!