Think there was a recent related support case to this …
A fix was checked into the 2.1 Enterprise branch to close a support case related to session tracking / conflict within a virtualized site. We believe that this fix also enabled support of Struts, though we have not yet had the time to rigorously test.
If you are a customer, you can download 2.1 Enterprise BETA 3 and re-test. Struts *should be* neatly supported and you should not see this error. Otherwise, you can either (a) request an eval copy of 2.1 Enterprise or (b) wait until an updated Community release in a few weeks.
Cheers.
Kevin