This is similar issues as described in issue #3651, but occurs when using forward slash '/' within alertmanager name.
Actually the fix PR #3664 for issue #3651 introduced regression.
Creating silence fails with 404 "Not found" error.
What to expect?
Silence is properly set.
Problem doesn't exists in karma v0.92 and below.
Problem exists in karma v0.93 and above (up to current last version v0.108).
Problem can be also resolved by using for example spaces instead of forward slashes within alertmanager names.
This seems to be due to go-chi/chi#641
RawPath usage in chi means that a request for
/alertmanager/foo%2Fbar/ doesn't match alertmanager
foo/bar since we'll have
Path: /alertmanager/foo/bar/ but
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