Hi team, I've read a thread or two on here about how to effect the long DKIM TXT record entries but it does not appear to be working for me and I believe I'm going about them wrong. It cuts off after 255 characters. I am running version 3.0.5.4p9 on Ubuntu 14.04 (an upgrade is scheduled for next quarter, I promise).
The reason for your problem is the old ISPConfig version. If you can't update, then you'll have to modify the ispconfig code and database field to allow longer TXT records.