REDMINE - Errno::ENOENT in WelcomeController#index

No such file or directory - /tmp/mysql.sock

TRIAL 1
=======
In database.yml you delete all database entries beside your production site:
RESULT:
The page will be blank!

TRIAL 2
=======
Adding "socket: /tmp/mysql.sock" to database.yml (production)
RESULT:
No change

Trial 1 + Trial 2
=============
RESULT:

Application error
Rails application failed to start properly

Putting back all other database entries into database.yml (Redo TRIAL 1)
RESULT:
Same Error as at the beginning shows up

>> It seems to do something with the database entries

TRIAL 3
=======
Putting Password in database.yml in "password" with " "
RESULT:
No changes at all

TRIAL 4
======
Entering Data for dev database in database.yml
RESULT:
The complete REDMINE site shows up perfectly

TRIAL5
======
Deleting "socket: /tmp/mysql.sock" to database.yml (production)
RESULT:
The Pages still show up perfectly

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FINAL RESULT
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It seems like REDMINE needs at least 2 databases assigned.
The passwords and connection entries in each database are completly different
- different connection settings "mysql_***" as on servage you are using no localhost
- different databases
- same or different passwords (both working)