If you encounter an Internal Server Error when going to Users page in Salesforce Setup, it may be due to the Default Application not having been set on the affected Users Profile.
For example, a Salesforce admin may encounter this error after creating a new custom Profile and attempting to manage users — the error occurs because no Default Application (such as Sales or Service) has been set on the new Profile.
To resolve the "Internal Server Error" when going to the Salesforce Users page in Setup, perform the following:
Salesforce Classic
Lightning Experience
000391917

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