Hotfix 15 has not resolved it for me. I have no save files left on the PS5 (there is a small set, maybe, in a multiplayer game with my spouse.)

Some relevant numbers:
Console storage: 667.2GB
FREE space: 405.6 GB
Games & Apps: 234.5 GB. (Balder's Gate 3: 119.0 GB)
Saved Data: 160GB
The ONE remaining saved BG3 file is only 713.1 MB. This file is inaccessible to me because I haven't subscribed to PS's subscription-only cloud backup service. (For which, I have to be honest, this is feeling like a manufactured error to drive players to subscribe.) Even if I payed PS the fee to move that file to their cloud, I've found no description of how to get BG3 on PS5 to recognize and load it. Not that, right now, I would trust the game and console to do that seamlessly anyway.

What it did, originally, is throw out the Error 544 (Unable to save), and then, in the files listed was a brand new nameless campaign directory. This would contain my most recent saved file from my character's directory (as in, that newest save file would stop being listed under the character's directory and would now belong - in a grayed-out inaccessible state - in a new nameless campaign directory. If I attempted to save again into my character's directory, it would move the new top file from the character's directory into that nameless campaign's directory.)

Rebooted PS5. No joy.
Tried leaving it powered off overnight. No joy.
I tried cutting auto saves and quick saves to 3, then 2. No joy.
Turned off cross-saves. No joy. (Deep sorrow, in fact, because now I don't have any Larian cloud saves for that campaign, afaict).
Briefly, for about three saves, I was able to complete a save after deleting two or three prior save files.

As of right now, the game is literally unsaveable. As in, today I began a brand new campaign, thinking this was a problem only in Act 3 when the save files got to be too large.

Nope. Literally at the very first save point in the Prologue, Error 544 - unable to save. You can't tell me this is some kind of memory issue. There's plenty of memory available on my PS5. This didn't happen in late December, when I was completing my first playthrough.

Literally the very first save point in the Prologue. The. First. One.

Larian, please, please figure this out!