Im not able to read French error messages (I assume it is French). And i guess lots of other people here also are not.
So if you post your error messages, it might be useful to translate what is says.
You said you can open the file with your android.
Can you send the file to your pc (email, ftp, whatever) and try to open the file? Result?
And as the file is sensitive for you, im pretty sure you have a backup of it. Does this backup open on your pc without any problems?
Yes it’s french
First pic says impossible to open the file.
Second says cannot open file for different possible reasons ( name does not exist or file name incorrect, file used in another program, excel workgroup open elsewhere).
Already tried downloading the same file but could not open it with pc.
The main question is: why can I open it via my android phone and not through my pc ?
You can try the 1.4 Beta that is linked in the post above and use dockany instead of Windows WebDAV as network client. Or you disable windows upload center.
It ONLY occurs when the PC does not have internet connection.
The vault can be mounted. Office files cannot be opened unless the PC is online.
BUT files can be copied to a local unencrypted folder and opened without issues.
Excel is latest version of 365 Click-and Go install (Versoin 1903) 64bit.
Windows is Win10 x64 Enterprise 1803