Mohr Siebeck, one of wikipedia library's partner, changed how their website works.
Here are the steps to reproduce
1. log to mohr siebeck using a wiki account, example https://www-mohrsiebeck-com.wikipedialibrary.idm.oclc.org/buch/deliver-us-from-evil-9783161573439/
2. click fulltext/volltext
3. get unauthorized IP error
Why does that happen?
Mohr Siebeck started to encode the downloader's IP in the download url, this was not the case until couple weeks ago.
The problem is that the download url does not pass through the wiki ezproxy, so it will be opened by the user's private IP, causing an IP mismatch, since the IP authorized to downloaded the content (the one hardcoded in the url, which is fundamentally wikipedia ezproxy IP) will not match the IP accessing the url. Unfortunately the wiki ezproxy is not configured to work on the domain used by Mohr Siebeck content delivery system, which would be something like this
https://viewer.content-select.com/pdf/viewer?ip=132.174.252.114&id_type=isbn&identifiers=9783161573439&signature=086d00813328e93f4155452981e6f5b8b99a0fc3&session=00000000-0000-0000-0000-000000000000
So the issue is not on Mohr Siebeck itself, but on wiki's side. You guys have to configure the proxy to be able to work on content-select domain. On their side, Mohr should change their website to generate proper wikimedia-ezproxy compatible urls.
edit: upon further testing it seems the IP mismatch is not issue, but rather that wiki lost access to Mohr's ebooks? I could access ebooks as well as journals 1 month ago. But this is strange, because books are given as accessible, as the zum volltext button shows
https://i.imgur.com/FjrK4V1.png
When an institution lacks access to a content it should look like this
https://i.imgur.com/QEJUteb.png