When opening an InDesing file with the new 2026 version, the file opens without any issues at all.
No missing links, no missing fonts. Opens OK.

When trying to save that file, which was created by previous InDesign version (2025), InDesign encounters an issue while saving. A pop-up asks if I want to "Repair the file". I hit the "Repair" button. InDesign app closes abruptly.

So, I used another way to save the file without 'Saving As', nor replacing the file.

I opened the exact same file made with a previous version of InDesign, hit 'Save as', select IDML, and the app didn't crash. I close the InDesing file. I erase the InDesign file from my folder.

Then:

Opened the IDML file, hit 'Save As > InDesign 2026'.
The file saves as InDesign.
No pop-up.

I close the IDML file.
Open the recently saved InDesign (2026).

No more issues.