Can't connect to any server

Xfce4-dict (version 0.8.8 and previous) can't connect to any server. I always get the message: "Could not connect to server." There's no output to stdout.

There's no problem with my internet connection and the servers are online. I opened the corresponding port 2628 in my firewall for outgoing connections (TCP and UDP), tried it also with ports 80 and 443. I'm using IPv4 and (should) have IPv6 disabled, at least I only have an IPv4 internet connection.