Hello All,

I have stumbled into this issue myself on my desktop with Unity 2022.3.4 and here's what I did to fix it (as of 9/6/2023):

1.) Close your current session of Unity.
2.) Go to Blender's website and download the latest version (usually for Windows).
--NOTE: You should be downloading the regular promoted Installer; the portable installer is the zip with the blender-launcher Application file mentioned in Etienne's link, which WILL NOT get recognized as an actual Blender install by Unity should you try to do the "Open With..." using the Blender 3.6.xx blender executable from the blender-launcher file.
3.) Once you got your Blender 3.6 (or 3.0+ generally) installed, you're going to navigate to the Asset directory where your broken .blend is located in your Unity Project folder. Do this in your (Windows) File Explorer.
4.) Left-click once on the .blend file to highlight-select it.
5. Then, hover over the "Open v" small icon next to "Properties" in the upper right of the File Explorer header Tool bar.
6.) Drop down the list and select "Choose Another App".
7.) Select the appropriate Blender install (which will normally just appear as "Blender" application). Check the box "Always Use This App" at the bottom of the dialog box.
--NOTE: IT's advised you figure out in advance which is the correct 3.0+ Blender by testing the "Choose Another App" to open the model with leaving the "Always Use This App" unchecked and viewing Blender's version once the program is launched.
8.) If the modeled opened in Blender 3.0+ successfully, then you're good to go from here on out.
9.) Re-open your Unity Project (delete the Library folder for a full-reload) and then once in, go to the .blend model that's broken inside Unity and now click "Re-import".
--If correct, Unity should spit no error and your model should load.

These are updated steps post when Blender now default installs to the restricted %AppData% directory that a normal internal Open With... can't access which is why we had to use the File Explorer's "Open v" then "Choose Another App". This also rendered the blender-launcher bush-fix from Etienne's thread usually ineffective since the blender-launcher location in C:/ProgramFiles no longer exists for the up-to-date Blender wizard install (again, it's in %AppData% now). Additionally, Unity will use the OS default application association with .blend files so a PATH Environment Variable fix won't do the trick. Thus, we had to manually trigger an embedded "Choose Another App" to get an "Always Use This App" checkbox to perform this OS default application reassignment for .blend files.

Frustrating I know, but hopefully this one comment will reach more people who come into contact with this issue later down the line.

I don't know why it is closed, but this issue is still present in Unity 2021.3.20 and when using blender 3.4.1

Why this is marked as resolved for 2021? IT IS NOT FIXED IN THE LATEST 2021.3.16! Project is COMPLETELY BROKEN BECAUSE OF THIS.

Was hoping the newest unity release would fix this. Still broken! Please fix ASAP, many indies use Blender as their modeling software for Unity projects!

this is a pain for me as i have a lot of blend files in my projects that i want in other unity projects

This is a pretty desperate issue for me!