Returnal's first post-launch patch has had to have been removed and itself patched following the discovery of a nasty bug that appeared to be corrupting players' saves.
Patch 1.3.3. was released as promised yesterday evening and was intended to resolve issues with certain trophies not unlocking, stability fixes and more. However, reports swiftly came in of the aforementioned save corruption bug, and it was pulled a couple of hours later. Now, developer Housemarque has issued a new patch known as 1.3.4 which will—for the meantime—revert the game back to 1.3.1 while still addressing the trophies issue and also fixing a keyboard exploit.
The studio has reassured players in the update post that with the new 1.3.4. update, all save games are safe to play now, but if you've already been affected by the bug —in that you're getting a CE-100028-1 error popup—then you have several ways to proceed open to you.
If the corrupted saves have maintained their file size, you should be fine to continue after the new patch. If your saves are suddenly reporting as 0KB it seems those save games cannot be fixed by the patch, which means you have two options; either you can restore a save from PlayStation Plus' Cloud feature, or sadly you'll have to start over from scratch and progress will remain unsalvagable.
It's not an ideal set of circumstances and as you can imagine has drawn a fair amount of ire from the community. Still, Housemarque say they are already busy on Patch 1.3.5. and the studio hope to avoid a similar issue in the future. We hope so too, because we think Returnal is a must play on the PlayStation 5—check out our review over here.