Hi sveijk,
I've been using the Mac Citrix Client version 6.0 (Build 96) for a while without any problems under X 10.0.4 Classic. Yes, even the video streams correctly now with this new build (took them 4 months to fix this from the v6.0 B66). The nice thing about updating to this version is that you can keep your old version document which has your specific settings. I would suggest trashing your Citrix preferences, the whole folder, regardless of upgrading or not.
Citrix Java Client - I've installed it for X and found it to be a bit cumbersome to initiate a connection. One has to cd to the Citrix directory via the Terminal, launch the executable via the Terminal(that you must make yourself), wait for the JVM to load (if it loads properly the first time), then manually enter in your client settings in the connection window (IP Address, caching, window params, colors, home directories, etc.), then finally click Run to start a connection. Not only is making a connection an ass grinder, but to have to basically make the application yourself. This is what I call poo poo.
All this ballyhoo seems a bit senseless. Takes computing back 20 years if you ask me. But we're talking about Citrix here, maker of software for all clients, master of none (hee hee). Well I might be a bit harsh on them but they do treat the Mac users like a step child.