To answer your first question: no, it's not a known issue. I don't know if it can be fixed. You can try proxying the requests to see if it's a problem with citrix or mendix (ie, whether citrix is messing up the requests or whether mendix is somehow borking them when they arrive).
You could also try different browsers, or delve into the citrix config settings and see if you can tinker there a little. I personally don't have that much experience with citrix, but you might check out how it deals with timeouts between the client and the citrix server.