It has nothing to do with SQL.
It's a problem with your application configuration file.
Try to clear down \bin\Debug and \bin\Release folders.
After, build check what config file is copied there and its content
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"The only true wisdom is in knowing you know nothing"
"O skol'ko nam otkrytiy chudnyh prevnosit microsofta duh!":-D
(So many miracle inventions provided by MS to us...)
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