Can't backsave Layout 2020 into Layout 2019

Hello Sketchup Community!
Please help!
I am trying to backsave a Layout 2020 file into Layout 2019.
When I then open Layout 2019 and my backsaved document, BUGSPLAT!!! - every time! Very frustrating!
Have tried backsaving as 2018, 2017 and nothing works.

Look forward to hearing your suggestions and if I am doing something wrong.

We want to backsave into 2019 as we are having issues with Layout 2020. It is super slow and lags/freezes often.

Can you share a layout file that’s crashing 2019 for you, to see if it’s your Layout 2019 that’s wrong or the file saved from your Layout 2020. If it’s happening every time even one of the standard templates would do, backsaved to 2019 in your Layout 2020.

I tried exporting a standard template into 2019 and this worked.
I then deleted all but one page of the Layout 2020 file I am working in, and this also opened with Layout 2019.
I then backsaved another document that is 45mb and this opened with 2019.
So I think it is a problem with my Layout 2020 file which is 93mb.
Not really able to send you the Layout file as it is a private home etc.
Is there a way I can check if the file is corrupted? Or could the problem relate to the file size?
Thank you