Cannot save "filename.psd" because of a program error - since Update 14th Dec 2020

Similar problem to may others. I use Photoshop 2021 with CC, with Macbook and Catalina and never had any issues in all my years of PS use, but since the update a few days ago, I cannot save files at all. this happens if:

-I have multiple layers

-I have resized an image (and during the image resizing process as well!)

-I have used a transform tool (rotate, resize etc)

I note the workaround that some people have found where you can put all the layers in a folder, set the folder to not visible and then it will allow you save as .psd, and this works for me, but I cannot save it as any other file type, and of course if you save it as jpg/png with the layers hidden, it saves as a plain white page.

I have a client who's ordered a Christmas commission, and I cannot send them the file!

I have tried:

-uninstalling and reinstalling Photoshop

-downloading an older version of Adobe Photoshop 2021 (the oldest version I could get on CC was 22.1, but still had the same issues as the latest version.

-copy and pasting each layer onto a brand new document and trying the save that (doesn't work)

-deleteing the original document in my files (as some mused it was a temporary file issue) and trying to resave (this also doesn't work)

I was thinking to try and download the Photoshop 2020, but there's no option I can do this from CC and am struggling to find it on the Adobe site.

An advice much appreciated!