I would think that forum is rather for discussing Airport/Airport extreme issues, both of which I don't have.
It's only this one computer running 10.6.8 I'm having problems with, hence I chose this forum and added the "getting_online_and_networking_macosx_v.10.6" tag...
But I'll give it a try anyway, thanks.
Try the basics first:
Change your router channel number. Most times this works & is all you have to do.
Disconnect & reconnect your modem. unplug it for about 10 seconds. Plug it back in. Do the same for Apple’s routers. Wait for everything to reboot.
Click the Assist Me button.
In the next window that pops up, click the Diagnostic button & do the necessary.
Research Knowledge Base for network problems that pertain to the OS that is currently installed on your computer. See these basic networking KB Articles: http://support.apple.com/kb/HT1401 AirPort troubleshooting guide
http://support.apple.com/kb/HT2712 Using network locations in Mac OS X
http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting
Thanks, Baby Boomer.
However I've been through all the basics and all the more advanced things I could think of.
As I wrote, the network is perfectly fine for two accounts, but not for the others ones (including newly created ones), so it can't really be a problem of the connection itself, but must be something account/OS specific.
Well, I think there were quite a few details in my original post that clearly indicated I'd put some effort and some thinking into solving this problem and that it can't be connection-related. If the only advice you can give is "just take it to the Apple store", I really wonder why you cared to post at all?
I posted because I didn't know if you have tried everything possible including the information in the links I posted. Plus, I though I was helping you. Was I wrong? If so, sorry about that. Will abandon the post.