I would start completely over. Wipe the Pi, reset the DSL modem and start fresh.
I understand that you'd like to block every and all advertisement on the internet and in order to achieve this I think you've added every blocklist that you could find online. From what I understand is that this also has blocked certain domains causing a password reset link to not work anymore. This is probably a result from all the 3rd-party blocklists blocking services by accident that your Gameshop uses.
Some ads on the internet can not be blocked. Really simply put, YouTube mixes the advertisement data with the video data so Pi-Hole can't discriminate between the data coming from the same domain.
Other apps and services are starting to catch on and will try to do this too, like eBay. After running a Pi-Hole on my home network, my VPN and somewhere else I can tell after a full year of use, it blocks 85-95% of all ads. I have never added any other blocklist. I only use those default in the installer and they have never given me any problem with sites not working or services being blocked.
Some ads will get through. 100% blocking might be possible but it will require you to understand Pi-Hole, its working principle and the relation with the services and used blocklists. It would also need you to edit block files and for instance unblock the domain/service that your gameshop uses for the password reset. Which in turn requires your knowledge to find out which service is being blocked. For me personally I could do that but I know it's a waste of valuable time to manually curate those lists for my situation if it could go from 85%-95% ads being blocked to 90%-95%. It's only going to be a marginal improvement for most people.
Seeing the problems you've had in the past I'd recommend forgetting it and just sticking with the default blocklists that come with Pi-Hole.