[petrjahoda]

So I tested:
- SAMBA 100%

- iCloud 100%

- Dropbox 100%

- Google Drive is behaving strangely, I selected it without problems, everything works as it supposed to work, but when I switch to dropbox and then back, even in this Google drive I now see grey button (when previously it was blue). Restart, reinstall wont help

- OneDrive is behaving with the same strangeness, I made restart because of that google drive, then selected it without problems, everything works as it supposed to work, but when I again switch to dropbox and then back, even in this Google drive I now see grey button (when previously it was blue). Restart, reinstall wont help

So I wanted to be sure the problem is not in the app, and tested other programs, that open folders (VLC, Obsidian, Texttastic) and all have the same problem.
What is strange is that it worked for me the first time I install it. But even after reinstall and restart the phone not just in justRead, but in all those other apps I see grayed Open icon.

By making a quick research over the internet it looks like Dropbox makes the integration for iOS the right way, but Google and Microsoft do not.

My guess is, until anything changes either in iOS or in GoogleDrive and OneDrive, it is not in my powers to do anything with this.

And this is the summary from google gemini (maybe explaining why it worked the first time after fresh install):
A greyed-out "Open" or "Save" button in OneDrive on iPhone usually means the app is in read-only mode, you are not signed in, or there is a file path error. This often occurs when using the Apple Files app to save files.

Common Fixes:
Use the OneDrive App: Open the dedicated Microsoft OneDrive app instead of the Files app to open/save files.
Sign In: If the icon is grey with a line through it, you are not signed in.
Restart/Reinstall: Restart your iPhone or reinstall the OneDrive app.
Check File Paths: Ensure the file name and path are not too long.

Why it happens:
Read-Only Mode: OneDrive for iOS may enter a temporary read-only mode to prevent file conflicts, disabling the button.
Connection/Syncing Issues: OneDrive may be paused, not signed in, or having connectivity issues.
Incompatible File Types: Some file types are unsupported on iOS and will be greyed out.

Similar goes for Google Drive.

I will test whatever I can to find out, why it worked the first time and what do I have to do, to repeat it, because now I am in the same boat, for google and microsoft, the button Open is grey.