Hello kind folks,
I've just upgraded a laptop from Debian 9 to 10 following this guide. Things went pretty smooth, and after upgrading I was able to boot Buster with no apparent problems.

Alas when I tried to access the internet, I found a problem I've never had before (and can't find much about on the internet, including this site):
When using Wifi, some sites work, while some will only make the browser complain it can't find the server. For example, I can connect to google.com, but not to duckduckgo.com. I can connect to wikipedia, but not to wikihow. Over LAN, I can access any webpage (afaict). Both regular users and root are affected. No firewall is running (that I can tell - I haven't set up anything, and flushing nftables rules comes up empty too). My router doesn't seem to be the problem either, as all other relevant devices can connnect normally.

I guess this is totally obvious noob stuff, but right now I'm at a loss as to where to look for configuration issues here. Any suggestion is welcome.
Thanks!

suggestion is to unplug lan when using wifi or vice versa?

^ This could be useful for troubleshooting at least.
Please try it.

Are you using network-manager? Delete all your connection profiles and start afresh. It just might be enough to fix the problem.

AFAICT?
It could be a IPv6 vs IPv4 problem.
Please ping all sites with the ping4 and ping6 commands respectively, share your results. Try to make 100% sure that only WLAN is affected.

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Thanks for replying!

The LAN cable was always disconnected when checking out wifi connections. Using network-manager, it seems safer to me to make physically sure which connection is being used.

I do use network-manager, and had already tried deleting and setting up a new connection, to no avail.

All sites I opened during my test were accessed correctly over LAN. I cannot exlude the possibility that I just got lucky and there are also websites that fail over LAN. But so far I didn't find any.

I think you're on to something here.
1. over WLAN

• sites that won't load: ping4 -> "connect: Network is unreachable" / ping6 -> "No address associated with hostname"
• sites that will load: ping4 -> "connect: Network is unreachable" / ping6 -> gets answer ("64 bytes from bla.com ...")

2. over LAN

• sites that won't load over WLAN: ping4 -> sending, but not receiving answer (71 packes transmitted, 0 received, 100% packet loss...) / ping6 -> "No address associated with hostname"
• sites that will load over WLAN: ping4 -> gets answer ("64 bytes from bla.com ...") / ping6 -> gets answer ("64 bytes from bla.com ...")

Not sure what conclusion to draw from this...

Not gaining any ground here, plus I noticed another weird problem (screen stays dark after display goes into power-safe w/o any way to reactivate). Since I don't have the luxury of surplus time to play around with the smaller mysteries of life, I'm going to call this system fubar and will try a fresh install.
Thanks for trying to help me figure it out!