Since updating to TFS 2008 we have lost our Sprint Burndown chart in eScrum, not a major problem as we use the cumulative flow in its place. However, I have eventually got round to fixing it.
It turns out the problem is down to the the way the dates for the start and end of the Sprint are converted to measure names for the main MDX query. There were both regional date format issues (mm/dd/yy as opposed to dd/mm/yy) and the fact that the MDX query was very particular over leading zeros for the end date of the range e.g. 14/4/2008 did not work but 14/04/2008 did (but this was not the case for the start date!)
The solution was to handle the dates as strings which I could force for format on, as opposed to dates. I am not sure how well this will work for others depending on regional date format. You might need to edit the RDL file a bit. However, my guess is it will cure many of the problems with missing eScrum burndown charts that forums have been putting down to warehouse corruption.
I also added a table at the bottom to help with seeing what is going on.