Failing RAM? In this economy?
Now to go beg to the RMA gods
Just to be clear: are you trying to add the new set of two to the existing set? I.e., 4x8GB?
That can sometimes cause problems due to timing variations between batches. Make sure to memtest each stick individually, to make sure that isn't the issue.
Edit: just realized that you implied that this setup has been working since 2024, so you can probably ignore my advice here