Issue: 

IDKit SDK running in the Internet Information Services on Windows Server returns error "IENGINE_E_PLUGIN_UNKNOWN (1301): IDKit plugin unknown error (see log file). "

Cause:

A different Identity of IIS worker under which IDKit runs could be a possible cause of the issue.

Solution:

Change the identity of to the default, i.e. ApplicationPoolIdentity. For further explanation see 

https://docs.microsoft.com/en-us/iis/manage/configuring-security/application-pool-identities