Notices

If I remember my windows ports correctly, 5800 is the browser port, 5900 is the vnc port, and 5500 is for 'listen mode' on the viewer.

My network connection is working fine (pinging works) and the subnet is the same, this is not the issue. Maybe I should clarify that I do work in IT. I'm just not that familiar with Linux.

I was wondering if could have something to do with rights, since I need to use sudo just to ping. Again, I don't know much about Linux, so I have no idea.

Thank you for you help anyway.

I am having the same issue, but only when trying to use VNC to connect to a Windows machine running VNC server.
Connecting to a Mac running its built-in share-screen function works fine.

Sorry Sasler; didn't mean to imply anything. We've had a lot of relatively nontechnical users show up lately so I've been trying to explain everything as 'least common denominator' as possible.
I doubt it's a permissions issue, but I could certainly be wrong.
I did a bit of searching around and it appears that the Maemo-Extras repository has a telnet client. If I understand correctly 'Extras' should be relatively safe to use, but you should probably avoid Extras-Testing and Extras-Devel at this point. To add the repository:
Go into the Application manager.
Tap the menu -> Application catalogs…
Tap Maemo Extras.
Uncheck Disabled.
and then you should be able to browse for it. I can't actually find it in the list, but a blog referenced it being there.
Additionally, this telnet client from the garage _might_ work.
Of course it's possible that there's a problem in the version of vncviewer you're using. Was it in the default repositories? I know there are several people who are apparently using vnc from the 900, but I don't know where it came from. Anyone with an n900 and working vnc care to speak up?
As a temporary alternative, I believe there's an rdesktop client available as well since you're working with Windows machines.
Oh, here's a link to a vncviewer that's supposed to work.
I wish I could be of more assistance. :/ I'll browse around a bit more later on to see whether I can find anything. Meanwhile, maybe see if that telnet client will install for you, or try that vncviewer link to see if it will cooperate any more.

Just for the sake of completion, what version of realvnc are you using? And what are your authentication/encoding settings at?

It's quite aright, I understand that. I wasn't upset or anything. I just thought to save some time by telling that I do understand the basics.

I cannot find the Telnet anywhere and the link you provided had only the .deb files which I can only save but not open. Any idea how to install .deb files? And is it safe?

Yes, the VNC Viewer is from Maemo-Extras repository. It's the same as in your link.

Thank you for your help.

What VNC server are you using?

VNC Server Enterprise Edition E4.4.0 32-bit
Authentication: VNC password
Encryption: Always on (I've tried this with "Prefer off", but it didn't help)

Yeah, I was thinking that maybe the vncviewer didn't have encryption support, but apparently that's not it (or at least not all of it).
Well, I'm not entirely sure how safe it is. I've never managed to brick my n810 by installing arm .deb packages, but I can't necessarily promise that they won't break your n900. I would assume that in the case of a telnet app the worst-case scenario is that it simply won't run. What I would probably do is try too install it and if it says it's going to need to remove or upgrade any other packages, say no (it'll prompt you if it thinks it needs to). It _may_ not be 'optified', so it might use your root partition for storage, but it _should_ be pretty small. Here's the deal:
1. Download the armel .deb file to your tablet.
2. Open an xterminal.
3. Type this and hit enter:
find / -name telnet-client_0.17-36_armel.deb
4. That should show you the path to where it's stored (for example, /home/user/telnet-client_0.17-36_armel.deb)
5. Type the following to install it, substituting the full path name for _pathname_:
sudo dpkg -i _pathname_

If the package doesn't require any additional dependencies, it will just install. If it does, it will tell you what it wants to install/remove/upgrade and prompt you Y/N. As long as you don't remove a bunch of packages on accident (by saying 'Y'), I wouldn't think that it would be terribly risky-but I can't say for sure without trying it out. If you can wait a few days, my n900 apparently shipped today and I should have it by the end of next week; I'm more than happy to test it out on mine first. VNC is going to be one of the first applications I install, and I'll eventually need telnet anyway. I'm happy to keep trying to troubleshoot with you, but without something (telnet) to try that port from the tablet I'm sort of stuck; it sounds as though you have everything in order over there.

Yeah, I think I can wait.

Tcpdump could give some indications what is happening, but I can't find that either. Strange. I hope there will be soon an easy way to install these basic tools on N900.