ISPConfig panel is slow with IPv6, update script not work

Hi,
when I set IPv6 in /etc/network/interfaces the ISPConfig interface become very slow. There is loading image visible for more than minute. Moreover ispconfig_update.sh returns:

Code:

Unable to retrieve version file.

Both works well before adding IPv6 to config file. I am on debian jessie.

What could be the culprit? Is there some way to debug?

Thank for any idea.

 

Hi Till,
you are right. There was problem with firewall. Thank you.