I just had exactly the same issue - flying EAU-MSP-ATL tomorrow with the second leg on DL and errorred out when I tried to check in on Firefox but worked OK in IE.

No hint of an upgrade yet as a NW plat. It does show 'Requested' for the upgrade when I look at the reservation at delta.com.


Note that if you have a 012 ticket with DL flights, you can use the confirmation number to look up your flights on DL.com and change seats etc for the DL flights that you can't change on NW.com.