Models from blender 2.79 - bug coverting into fbx

Using 2.79 blender for making models. The Unity earlier easily imported new changes made in model. Now if Unity sees the model, when I open it in Blender, make edition and save, Unity looses it, writing missing mesh and message (in red in below) that it can not convert blend file into FBX.
Staring experiencing problem 2-3 weeks ago with 2019.2.1 version. Still experiencing.
Any idea how to fix?

Don’t think anything changed regarding fbx in the last weeks. Is it possible you installed blender 2.8 in the timeframe and now all .blend files automatically open with that instead of 2.79? If so try linking 2.79 as standard program to open .blend files again.

please tell how to teach Unity to do that!!! thank you

I mean the windows standard program to open it. I think unity might start that in the background to convert?
Right click on any blend file and change what opens it under Open with.

thank you, will try now!