This thread discusses the same issue, you may find some answers in there: http://world.episerver.com/Modules/Forum/Pages/thread.aspx?id=49933
If I understand correctly the XForms task is run after the actual upgrade so everything should be OK except for your xforms data. It is supposed to do some kind of unescaping of html or something like that.
A colleague contacted EPiServer dev support about this but they seemed unaware of the issue and offered no solution to the problem other than deleting all xforms and start fresh...
What we did to fix this was to connect our database to a standard CMS 6 site and upgrade that instead, in which case the error didn't appear. Then we manually did the file and config changes in our site (which we would have done anyway since we use a lot of different configurations and transforms) and reconnected the DB.
We attempted to upgrade a v6 R1 site to R2. Everything seemed to be going smoothly when the upgrade threw an Exception when renaming the XForms. The upgraded terminated (one assumes prematurely).
The site comes up, and appears to be R2 (I have Visitor Groups). There are some oddities in the templates (things aren't showing that were before), but it seems functionality.
I attempted to run the upgrade again, but it complains that the site is not the right version. I assume this is because it's identifying itself as R2 now, and therefore claims to not be in need of an upgrade.