I had the same.
It's a bug with the new Go-based symbiosis-httpd-logger that replaces the previous Ruby version in the most recent Symbiosis Jessie packages. I have a pull request outstanding to fix this https://github.com/BytemarkHosting/symbiosis/pull/10).
I'm now running a locally updated Go logger. An easier fix for the moment would be to switch back to the old Ruby logger. I've not tested this, but I think:
# rm /usr/sbin/symbiosis-httpd-logger
# ln -s /usr/sbin/symbiosis-httpd-logger.dpkg-symbiosis.orig /usr/sbin/symbiosis-httpd-logger
should do the trick. Apache restart needed, obviously.
EDIT: On reading another topic, instead of above I think
$ sudo apt-get remove symbiosis-httpd-logger
$ sudo service apache2 reload
may do the trick.