This is very complicated to describe. I'll try to describe our setup, our Jenkins configuration's chronology the problem and my assumptions to the current bug...
I'll describe the whole situation which probably makes the description harder to understand, but I want to provide as much information as possible. Please help me to make this description more specifcally and/or create another issue for separate problems. Thank you.

Setup
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Jenkins configured to authenticate through Crowd 2 Plugin (works fine so far, also with authorization)
The authenticated username is Forename Lastname (crowd-userid). Crowd holds the crentral email address of each user.
Out projects are checked out through Git. Older commits may have malformed auther name and email. Recent commits are all in form Username = 'Forename Lastname' and email = '<The same email address as in crowd>'.

Cronology
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• This Jenkins installation started with no security configured.
• I can't ensure there was never a Jenkins-based usermanagement configured as this installation was administered by a colleague before.
• All jobs check out from a Git-Repository
• Before today the "Peoples" view showed a variety of user id from more or less real "names" (also misleading names like "root", see my assumptions below) over "" through odd usernames like "contextMenu" (we can't figure out what caused this username).
• Some users seemed to appear twice (this is actually another issue)
• Today I've configures authentication through the Crowd 2 Plugin
• Now there are further users, one for each Crowd user that has logged in once

Problem
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There are at least 2, sometimes 3 users (obviously depending on how much different Git users exists) in Peoples view. Only one of them, apparently the user generated out of the Git user can show information under "Builds"

Assumptions
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• A username is generated for each user of a Git commit
• If there was accidently no username like "<>" or "login <login@hostname>" or "root <root@hostname>" such user IDs ("", "login", "root") will be generated

So how can I...
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• delete these accidently created users
• tell Jenkins to see/understand <automatically generated user x> is equal to <Crowd user X> or at least map each Git user to a Crow 2 user
• keep Jenkins from generating user IDs for all Git users

I know there are other issues and solutions on the web about removing users, but deleting in users/ did not work either. All user-directories are re-generated on Jenkins startup (all having statup date).