Question: There it is, once more:

```
The process for the file protocol died unexpectedly

```

in red! Dolphin doesn't show files, except of the current directory before the death. Closing every instance and restarting doesn't help. Starting from command-line shows:

```
QSqlQuery::prepare: database not open

```

when the error is displayed. And when clicking on some favorites:

```
QSqlDatabasePrivate::database: unable to open database: "unable to open database file Error opening database" 
QSqlQuery::prepare: database not open

```

It works for ssh-fs links (fish as protocol)... thats suprising.

I hope it's not related to the nfs directories... but that would help: what is the reason for this crash? Is it possible without reboot / logout and -in to get dolphin running again?

Nautilus works fine, mc also. I'm running Debian Jessie (Testing) with Gnome 3. (And I don't like nautilus :)

Answer: 
This just happened to me in awesomewm after updating some KDE packages. When launching dolphin from a terminal, I got:

```
kf5.kio.core: "The process for the tags protocol died unexpectedly."
kf5.kio.core: "The process for the trash protocol died unexpectedly."

```

and nothing was visible in Dolphin (other than the same red error message as OP).

The solution seemed to be killing `kdeinit5`.
Well....it might be time to restore the database files from backups, presuming you do periodically make them.

A couple of other thought include package corruption, you could reinstall the old package over the current one. 

Second, what about a compromised server?