Apparently, ispCP has changed its repository URL (why :800? Think of the children^Wfirewalls!), leading to:

$ svn update
svn: Repository moved permanently to 'http://isp-control.net/ispcp_svn/trunk' ; \
please relocate

Luckily, svn switch is here to help, the magic command to resolve this one was:

$ svn switch --relocate \
  http://www.isp-control.net/ispcp_svn http://isp-control.net:800/ispcp_svn .

$ svn info | grep -A1 ^URL
URL: http://isp-control.net:800/ispcp_svn/trunk
Repository Root: http://isp-control.net:800/ispcp_svn