I don't have a xoom, but I do have two other android devices with very similar problems with both Lotus Traveler and the Domino webmail. I have a Razr Maxx phone running the stock Gingerbread OS and it has no problem with Traveler or webmail. A few weeks back I flashed a leaked ICS ROM and neither Traveler nor webmail worked. Traveler installed OK and allowed me to log into server, but would not sync messages or calendar or anything else. The webmail would fail with the 500 invalid post error when I tried logging in. After playing around with it for a few days I reverted back to Gingerbread and it all works.
I also have a Kindle Fire that was on CM7 Gingerbread ROM and it had no problems with Lotus webmail. I don't really use it for work and don't want to install Traveler on it, but it was handy being able to get to my work email if I needed to. Then I flashed an ICS ROM and webmail gets the 500 invalid post error when I log in just like the Razr when it was running ICS. Since the KF is primarily for reading books and news, I am sticking with ICS. But it's inconvenient not being able to get to work email.
So in my experience, going to ICS seems to be the thing that breaks webmail. The odd thing is that I'm using the same browser (Dolphin) both before ICS when it worked and after ICS. I have tried other browsers on my KF including Firefox and the stock browser, but none of them work. When I was on GB all browsers worked. What is there about ICS that can change the way data is posted to a web site? Or for that matter the way data is sent by the Traveler app? I wouldn't have thought that the OS version should come into play, and for what it's worth, I haven't seen any problems with any other web sites.
I'm really curious what's going on, especially since rumor has it that Moto/Verizon will start pushing ICS to Razr Maxx some time in the next month, and I fully expect to lose Lotus traveler since it only applies to Lotus app/web site and there are not that many users out there.
Any ideas or suggestions would be appreciated.