Hello,
The recent fix for GEOS-7753 (unable to save stores) has broken the cascaded WMS store.
The error seems to be a simple typo. I have fixed the typo locally and am looking into test cases. See comments on the commits below:
2.9.x: https://github.com/geoserver/geoserver/commit/bb8be8fee2565d8ea1886686ebcab7f7df292c48
master: https://github.com/geoserver/geoserver/commit/79fd8dba4ecef792bd0f56ce47da7b8d6dc6b0b0
Torben