If you are getting the following exception when running SPSiteDataQuery, you may want to check if you have any of the following three fields types in your <ViewFields> element.

1. Lookup field
2. Person or Group field
3. a field called EventDate

The root cause for the above three fields is that the back-end query into SQL, that needs to return one than one columns for any of the three site fields(columns), is constructed to return a few more other columns from DB. However, the Nullable setting only mark one of the many fields into allowing NULL values. There is nothing we can do on # 1 or 2, but for #3, we will have to add a few more fields to avoid the problem.

<ViewFields>
<FieldRef Name="EventDate" Nullable="TRUE" />
<FieldRef Name="fAllDayEvent" Nullable="TRUE" />
<FieldRef Name="EndDate" Nullable="TRUE" />
<FieldRef Name="TimeZone" Nullable="TRUE" />
<FieldRef Name="XMLTZone" Nullable="TRUE" />
</ViewFields>

Please see the following Knowledgebase article on the limitations of the SPSiteDataQuery.

http://support.microsoft.com/kb/946484

Here are some links to SPSiteDataQuery samples

http://blog.thekid.me.uk/archive/2007/02/27/spsitedataquery-samples-for-wss-v3.aspx

http://blog.thekid.me.uk/archive/2007/02/25/xml-results-using-spsitedataquery-in-sharepoint.aspx

If you want to return the built-in EndDate field (not to be confused with "_EndDate") you will also need to return the group of fields listed above. Otherwise, SP will throw a COM exception which doesn't point to the source of the problem. I.e.,

<ViewFields>
<FieldRef Name="EventDate" Nullable="TRUE" />
<FieldRef Name="fAllDayEvent" Nullable="TRUE" />
<FieldRef Name="EndDate" Nullable="TRUE" />
<FieldRef Name="TimeZone" Nullable="TRUE" />
<FieldRef Name="XMLTZone" Nullable="TRUE" />
</ViewFields>

Interestingly, if you use "_EndDate" instead of "EndDate", your query will return rows instead of blowing up but the EndDate will always return an empty string.

See the post "SP Field Dependencies" at the bottom of http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spbuiltinfieldid.enddate.aspx for why all 5 fields are required.