Figured out the problem.  It was not the column groups at all.   Just hate it that VS with Data Tools makes it so easy to miss the real problem.

Come to think of it, I doubt it's the version of SSRS.   If you put an image in a header, it's not going to change until the group the header...

Check the permissions on that folder that the error identifies.   Be sure the security context under which that powershell script runs, has access to that folder.

Here's some data to back me up:
    If your stored procedure took 15 minutes to run, you'd be doing this for 5,707,031.25 YEARS.  (Clearly impossible to accomplish)
    If...

How about we consider the need for the result to NOT be NULL?
SELECT TOP (1) Field
FROM TABLE
WHERE Field IS NOT NULL
ORDER BY [Key];

Why do this?  What's the bigger picture?

Doesn't have to actually be that long.  Once you have the human readable string, here's the SSIS code to get it into a date:
(DT_DATE)((DT_STR,10,1252)REPLACE(REPLACE(REPLACE('0x323031342D30342D3037', '0x', ''), '2D', '-'), '3',...

Not sure, but have you tried making an explicit transaction such that the SQL command you supply does the BEGIN TRAN followed by an INSERT that includes your SELECT from...