Hello Andy,
in order to get better help you should provide more details:
what is the IOS version running on C800 router.
What is the SDM version.
if you access the C800 from telnet/SSH can you try to do the following:
close SDM application
get telnet/ssh access
enable
from router# prompt
issue
copy running-config startup-config
do you see error messages?
Hope to help
Giuseppe