...are there any log-files, output-files etc that would help to gather information....
1. Personally I use this
http://trac.netlabs.org/xwlan/ticket/17#comment:2 Rexx script which constantly monitors my LAN connections. I've this cli window open all the time when testing switching between wired and wireless LAN
2. XWLAN (both versions, widget and wlanstat) do have the menu option to display output from wpa_supplicant. So you see what the supplicant does. There are different verbosity options too.
3. If you activate the setting to get IP addresses automatically XWLAN3.12beta fires up dhclient in a hidden window. It's easily found in the xwp Tasklist and there you see dhclient log. But you can configure XWLAN to redirect the dhclient output to a log file too. In this case no window will be created but the log file written. Configuration for this is in XWLAN TCP/IP settings dialog.
Worth to mention - if you switch between wired and wireless interface XWLAN will let the wired connection up and running until the wireless is established.