I am having a reproducible situation where I click on the "Login to Control Panel" button in WHMCS which is supposed to use the session based login to go right into the Virtualmin Virtual Server Admin login area. What is happening from a freshly opened IE 9 browser is a 500 error is received after the first click on "Login to Control Panel". I then refresh the tab showing the 500 error and I see the login area. I then go back to my WHMCS Product Details and click on the button again, this time it logs me in without issue. Once I am able to successfully login it is difficult to reproduce the error.
I am also able to reproduce this in FF, but instead of a server 500 error I get an error about cookies until I successfully login manually with the username and password and then logout and try again from the "Login to Control Panel" button.
I have not been able to spot any errors in the Webmin logged actions.
Please let me know if there is a simple solution to this or where to look for possible causes.