(I'm using Tableau 8)
I am having a similar problem, unfortunately for me it seems intermittent because I've refreshed this extract (Excel) and it worked fine for over a month. The last time I refreshed it one of my group fields that I had created ceased to exist giving me the dreaded red exclamation mark.
Were you able to find a solution to your issue? I've tried to recreate this issue in a data set, but have not been successful - so I can't actually figure out what is causing me to have this problem. I just hate the thought that I'd randomly have to remake a bunch of these groups as it sort of defeats the purpose in this example.
Sorry this isn't an answer!
Has anyone found an answer to this?
My suggestion would be to always use calculations or some other data-driven method for creating your groups. The ad-hoc groupings in Tableau are not very useful, unless you only plan to use them for a one-time analysis. Just my two cents, but pretty much any grouping can be created via a calculated field, like (very simple example)
IF STARTSWITH([Name], "J") then "J Names"
elseif STARTSWITH([Name], "M") then "M Names"