I had a similar problem when I was trying to change seats on the 2nd leg of my trip before the 24 hour window (FO is 24 hrs - 12/14 to fly 12/15). I could check in and move seats for my 1 pm flight (at ~4 pm on 12/14), but wasn't able to get seat changes to stick on the 2nd leg (5 pm flight) until the 24 hour mark before the 2nd flight. FWIW I was also getting the same or a very similar error when trying to change seats via delta website.

The upgrade e-mails often come before the designated window for true OLCI and if it's a multisegment flight OLCI might also be before the window for the 2nd and/or 3rd segments.

Once I was checked in, I called the phone line and they moved the seats for the 2nd segment before the true 24 hour window (but they didn't know the problem at the time either). As FO, I don't get EC until OLCI so I wanted to grab EC on my 2nd leg - now that I've been through it once, I was able to figure out what happened and can "fix" it myself by simply being a little more patient

Hope this helps! P.S. I'm not sure this is a Bug per se, maybe just not so clearly documented/explained for the users. Once I figured out the timing part on the later segments (even though it was several hours post OLCI) it was sort of a d'uh moment for me.