I have used Inkscape for quite some time now, version 1.1.2 (I know, it's old 🙂) 

Now, having a new computer, I newly installed inkscape version 1.3, I can't open this file anymore. Can someone please help me?

It's a file I created with inkscape. 

I used to have windows 10 on the old computer, now it is windows 11.

There are a few possibilities. In my experience it's usually a font problem. Try using a text editor to open the svg file then change all the fonts to Arial or Times New Roman or some other commonly used font. I admit, this is tedious for large files. If it's not private, you could upload the file for examination.

Thanks for the tip! I changed all the fonts from Ubuntu to Arial, but still can't open the file.

I'm afraid its a private file, I can't upload it.

You could share it privately with Paddy_CAD.

So it's not a font problem, but there's obviously some other malformed svg code inside.

Here's another strategy. In the text editor you can easily see individual shapes within the svg file structure. Delete half the shapes, save the file and try to load the remainder in Inkscape. If the file opens successfully then you know the error is in the other half. Go back to the original and delete half the remainder. Repeat and repeat. By elimination you'll eventually identify the offending shapes. Usually there's just one but there might be more.

Thank you so much for your input! I'll do that when there's more time left over 😉