Symptoms

Server Error
Object reference not set to an instance of an object.
More Details
at SourceCode.Security.Claims.ClaimsResolver.ResolveClaimsToUserFullyQualifiedName(ReadOnlyCollection`1 identityCollection, IList`1 mappings)
at SourceCode.Security.Claims.SecurityClaims.ResolveClaimsToUserFullyQualifiedName(ClaimsTokenType tokenType, String tokenXml, Object issuers, Object claimTypeMappings)
at SourceCode.Hosting.Server.Runtime.HostSecurityManager.GetClaimsUserName(String tokenXml, ClaimsTokenType tokenType, ClaimsVersion claimsVersion)
at SourceCode.Hosting.Server.Runtime.HostSecurityManager.AuthenticateIIdentitySession(String sessionCookie, String tokenXml, ClaimsTokenType tokenType, Boolean isPrimarySecurityEntity, String authReqSource, ClaimsVersion claimsVersion)
 

Diagnoses

Ran a fiddler trace and tried to register the K2 for SharePoint on Office 365.

We navigated through the Fiddler trace and saw many queries against the K2 Settings list.
Then opened this list and noticed that I had duplicate entries for the entries called "SmartForms_Runtime" and "SmartForms_Designer".
 

Resolution

We deleted the duplicates and then proceeded to run the Registration wizard again, which resolved the problem.