Yes, but do you know a platform we could use?
I disagree to that: we should not discuss this in private, but in an open forum that can be found and used by as many users as possible!
i have basicly no idea, how device behaves behind a router with IP 6 internal and external IP nums and on top this might be very much rourter specific.
said that … maybe it’s a IP 6 thing. IP 4 would require NAT, but does IP 6?
second thought: maybe kleine anzeigen does some “profiling”? (screen sizesm, user name, etc.)
It depends on the provider settings. One would expect NAT on IPv4 and a routed public address on IPv6.
It is very much possible that they have some profiling system based on whatever the browser offer. There are several tests online to check one own’s browser fingerprint.
I found an appropriate forum:
It’s in German…
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One more thing: I copied my hole Firefox profile including addons to the live USB Linux and guess what? Yes, it works perfectly there! I can log into my account at kleinanzeigen without any problem. Even FF’s password manager works.
Maybe this information can be useful for some creative mind to think of some counter measures to this IP blocking?
Some entries in userContent or something similar?
I found a SOLUTION!!!
I just installed the FF add-on “User Agent Swicher and Manager” and set FF’s useragent to Chrome on Chromium OS. Et voila: kleinanzeigen works again!
I don’t know what they are smoking at “kleinanzeigen”, but it surely does not seem to be very healthy!
Sorry for having to bother you again, but things have worsened.
Switching the user agent no longer helps. If I do this, afterwards all browsers will provoke the blockage! 
I can still log in using my notebook or a tablet.
I can still log into that portal by using a live USB system, as stated a few days ago.
Derived from that possibility, I had another idea: I took a second hard drive and installed the latest Neon to it, that is available for downloading. Turns out to be the same version as my system tells me it is, which is no big surprise.
Right after installation I tried to log in to kleinanzeigen, with a virgin Firefox: no addons, no userChrome, no imported old data.
Guess what happened? Right: I was blocked! 
Doing a search on the net many users can be found, having the same problem. But only a very small amount of those people are using Linux, and yet even less users among those will be using KDE Neon. So, obviously, it’s not Neon that is causing this.
But someone suggested that the portal might use some sophisticated and faulty adjusted fingerprinting technique to identify the system they want to block. Taking into account my latest experiment, this seems highly likely. But if such a fingerprinting technique is involved, and if it recognizes a newly installed system wrongly to be the old system, what could the fingerprint it creates based upon? It can not be some simple sort of list of all installed packages, of CPU power, memory and other hardware stuff, because it that was the case, virtual machines running a different OS should not be affected, should they??
On the other hand: if this fingerprint was based upon the machine’s MAC, live USB systems shouldn’t work either.
Can anyone still make any sense out of this? I can’t!
Have you emailed them to ask why they are blocking you?
Yes, I have, but I do not expect to get an answer within this century. They have one online contact form for pro-users only, which I am not, and one e-mail address “service@kleinanzeigen.de”. If I write to that address using the same mail account that I registered with kleinanzeigen, I get a error message from some MTA on Google, telling me that the message could not be delivered, because my e-mail provide has a low reputation. 
Can you believe all this menace?
So I wrote to them, using a different mail account on a different provider. But I am afraid this will most probably cause too much confusion to them, so they most likely will delete the message.
Not all information about your system are made available to a website you visit. MAC addresses are local to the LAN and are not propagated through the network. The list of packages is very OS-specific and has little to do with the browser.
You should check potential fingerprinting attributes using appropriate online tools such as My Fingerprint- Am I Unique ?
Yet again, the process is unlikely to be deterministic. One can reasonably assume each attribute and/or combination of attributes contributes to a “score” that associates your system to the “banned” profile.
I’ve tried to register at <https://www.kleinanzeigen.de> from this platform –
Operating System: openSUSE Leap 15.5
KDE Plasma Version: 5.27.4
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8
Kernel Version: 5.14.21-150500.55.31-default (64-bit)
Graphics Platform: X11
Processors: 8 × AMD Ryzen 5 3400G with Radeon Vega Graphics
Memory: 29.3 GiB of RAM
Graphics Processor: AMD Radeon Vega 11 Graphics
Manufacturer: ASUS
The registration process was almost OK – e-Mail sent by “eBay kleinanzeigen” to confirm, with a button “E-Mail-Adresse bestätigen” – which seemed to work OK – at the Firefox Web-Browser, the registration tab changed to “Please confirm your telephone number – we’ll send you an SMS” – entered my pocket telephone’s number – more than once – even began a new registration procedure …
- I’m still waiting for the SMS with the registration PIN …
At a guess, eBay and “kleinanzeigen” have a problem which other companies here in Germany don’t have …
I would agree to this, but on the other hand: they force you to solve some crazy puzzle or some silly captchas, and the site complains when java script is disabled. I am not an expert on this, but if they force me to visit their site with activated java script and deactivated add blocker, wouldn’t it be possible to do the fingerprinting while I am forced to solve their silly riddles? And if this is true, couldn’t data like the MAC be included in this process?
Well, that page tells me:
Yes! You are unique among the 2143374 fingerprints in our entire dataset.
The SMS with the PIN arrived this morning and, 4 hours later once again – possibly triggered by one of the registration attempts … 
Whow! That was quick… 
What about different devices and/or different OS? Did you try any of these?
No, I haven’t tried with any other devices or OS …
Just to let you know: for a few weeks now, the login procedure nearly works as expected again: the credentials user/password can be typed in, and then you will be sent an e-mail containing a 6 digit code, that you will have to enter at the next page to verify your identity.