One larger customer recently started working with huge MS Cluster servers
and installed a citrix farm for all internal applications.

I've read in previous posts that:
The VFP database is not cluster aware.
What does this mean exactly because it seems people are still using it?

Is there any special things I can do to solve the strange situations that have been occuring there now?

Situation 1.
Some users run the application on the citrix and some run it over the normal network. We noticed that treing to open a Free Table from the normal machine worked fine but opening it from the citrix gives the error indicating it is open exlcusive. We had to kill all citrix sessions and it was fixed. This happens often.

Situation 2.
1104 Error reading file errors when the application is running.

Situation 3.
Corrupt FTP and CDX files on various tables. This seems to happen when the data is shifted to another server automatically while the application is running.