Invalid Argument when user tries to access CRM (On-Prem)

Very strange problem wherein one my users tries to hit the main CRM page, he receives a blank page with 'Invalid Argument' displayed.

The only tangible details I could retreive from the log are this:

>>>>>> EXCEPTION: 0
>Type: Microsoft.Crm.CrmArgumentNullException
>ErrorCode: 0x80040203: ErrorCodes.InvalidArgument
>Message (15): claimsPrincipal
>StackTrace: present: 91D8E22C
>no PluginTrace
>>>>>> EXCEPTION: 1
>Type: System.ArgumentNullException
>Message (54): Value cannot be null.
>Parameter name: claimsPrincipal
>StackTrace <null>
>no PluginTrace
>InnerException <null>
><<<<< EXCEPTION: 1

But I have no idea what it refers to. It seems to be an issue with my Claims Authentication but again, it is only happening to one user.