Whenever I open NotePad++ the following error message appears:

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Configurator
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That obviously means that there is also no syntax highlighting.

My OS is Windows 7 64-bit.

Somehow, your langs.xml has errors in it. Perhaps while exploring various Notepad++ options you accidentally made unintended changes to it. That's what happened to me.
Go to the installation folder for Notepad++ and rename langs.xml to langs.xml.bad. Then, in that same folder, find langs.model.xml, make a copy of it, and rename the copy to langs.xml. DO NOT simply rename the file or you will not have it available the next time you need it.
also replace this file in the folder C:\users\(user)\appdata\roaming\notepad++