Sessions on server disappear in CMC

[EGR]

Hi,

we've got an strange problem.
Environment:
-4x HP ProLiant DL380G3
-1x Compaq ML 530
-MS Windows 2000 Server, SP3
-Citrix MF XPa, FR2 (it was XPe eval. before)
-Datastore: Oracle 8.1.7.4 on SUN Solaris
-Printers: HP 2200, HP 8500

First there were three servers in the farm and about 70 people working on it (Office, Groupwise...) and there were no problems at all.
Yesterday we installed two new servers (DL 380G3) and joined the farm (we chose XPe during installation and changed the product code to XPa afterwards).
After these servers joined the farm sometimes strange display issuses happen in the CMC.
When we select a server in the CMC sporadically sessions and /or processes are not shown even though there are quite al lot of users working on the server. This happens on all servers but only after the two new servers joined the farm.
Before that there was no problem.

Any ideas?
Thanks.
/EGR