Users cannot start/stop a VPS

There are two possible issues here, often both occurring at the same time.

VPS information is not available from the DTC panel

If you get an error in the 'Installation' tab of a VPS saying that there is a SSL timeout, it likely means that the DTC-Xen daemon on the affected node is not running correctly.

Bring up a command line as root on that node and attempt to run /etc/init.d/dtc-xen restart. Hopefully this will bring up the DTC SOAP server. If this doesn't work, you might need to kill any existing processes so type ps aux | grep soap and kill' the offending process.

On some nodes, DTC-Xen is set up to respawn if it is killed (as opposed to being controlled by the init.d script). In this case, just kill any dtc-soap-server process (kill -9 if that fails) and hopefully it will respawn with a new process ID.

xm does not work

Try typing xm list to see if this will bring up a list of running VPSs. Do wait at least 30 seconds as this can sometimes be slow.

If this fails, the chances are that something is wrong with xend, the xen domain daemon. The primary cause is simply that the disk is full, so type df -h to check this is the case. The biggest cause of this is massive log files in /var/log so check these out my sorting by size (ls --sort=size -h | head) and deleting if necessary. It's worth noting that the system logger has a handle on these files so you will probably need to restart the logger to really free up the space. Try /etc/init.d/rsyslogd restart.