dns cname cannot be longer than 64 characters.
The interface refuses cname's with more than 64 characters. Unfortunately there a use cases that expect longer cnames.
Good examples are Amazon AWS cnames for s3 or ELB's.
They expect you to define cnames like:
www.averylongdomainname.com.s3-website-eu-west-1.amazonaws.com. dualstack.arandomlable-1397555634.eu-west-1.elb.amazonaws.com.
I have patched /usr/local/ispconfig/interface/web/dns/form/dns_cname.tform.php manually now ''regex' => '/^[\w.-]{1,64}$/',' as a workaround.
Hope you can increase the max value in the next release.