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User can access the website but is not logged in and the functions are unresponsive.
Under certain circumstances, the user can lose the Handler Mappings for *.svc files which results in the described behaviour.
Solution
Open a Command Prompt in Administrator mode and write:
On 64 bit OS write:
DISM /Online /Enable-Feature /FeatureName:WCF-HTTP-Activation45
On 32 bit OS write:
DISM /Online /Enable-Feature /FeatureName:WCF-HTTP-Activation
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