I assume that you have verified the server URL and user password.
To close it, you could run
from the command line, find the process number, then issue
kill [process number]
from the command line. Sometimes it can take a moment for the process to die.
I would suggest then starting pan from the command line; it might output errors to the terminal that would be useful in troubleshooting. Post the errors, if any, here, making sure to enclose them in "code" tags, which become available when you click "Go Advanced" down there at the bottom of the Quick Reply window.