I have been fighting this problem since the first releases of kde4.
I have made posts about it under different headings while not knowing enough to figure out exactly what was happening.
Unfortunately, this only happens to those of us who are using particular NAS drives on a local network.
There do not seem to be many folks on the forum who fall into that category.
My latest understanding of what is happening is that the KIO-slave that is part of kde4 opens too many connections to samba shares and is very slow to close them.
Many NAS drives have a limit of 5 connections at any one time.
As soon as the system tries to open more, you get an error of "server timed out" or "can not open " or one of several other messages depending on how you reached that point.
To see if this is your problem, open Konsole and enter the command "netstat -plant -c".
Now, access your NAS drive and watch the number of connections made. It you are using one like my Hawking drives, the system will start giving errors as soon as you reach five connections.
Even shutting down Dolphin will not close the connections. You have to wait for them to time out and close.
Some of them will not time out as long as you remain in Dolphin.
Many will time out after about five minutes.
As an example, I can access my movie shares on the NAS drive. I am up to 4 connections by the time I get to the movie folder.
As soon as I click on a movie, it goes up to 5 and still needs to open one more for the media player so the player gives an error message and won't run.
If I just sit and watch the konsole output from netstat, eventually some of the connections will disappear.
As soon as they do, I can click on a movie and it will play.
I went to the kde web site and searched on "too many connections" and found a number of posts on this issue as it relates to accessing FTP servers that restrict the number of connections made by a user.
The bottom line is that the developers are aware of the problem but have put it in a very low priority category.
Therefore, there is no telling if and when it will be fixed.
Yes, I know that they are talking about FTP servers and we are talking about Samba servers. But, it appears to be the same problem.
The first thing you should do is to go into the setup on your NAS drive and see if it allows you to set the maximum number of connections.
If so, upping it to at least 10 will probably solve a lot of your problem. Unfortunately, some units like two of mine have a maximum setting of 5 connection.
I have found several other things that help.
Go into Dolphin and disable the showing of the Information panel.
Also, disable preview.
Setting the NAS drive up with a static IP address makes a big difference.
If, by chance, you are trying to access multimedia files, Believe it or not, you can access them through Firefox.
You just type in smb://192.168.1.xx where the 192.168.1.xx is the actuall IP address of your NAS drive. It will allow you to navigate the file structure on the drive. When you click on a movie or other multimedia file, Firefox will play it using the mplayer plug-in.
This works even though you can not access and play the same file via dolphin.
If you look at the output of the netstat command while doing this, you will see that the access to the NAS drive is being made via some Gnome code rather than the kde code that dolphin trys to use.
This brings up the obvious that I have not yet tried. If you add the Gnome desktop to your install, you will probably be able to access the NAS drive with no problem.