Event Registration is failing on portal for all the events except one event. All the events are similar sample events created for demo purpose. Below is the plugin trace log which is not very helpful.
entPortalLinkPlugin.Strategies.PortalCGI.PortalCGIApiHandler, Microsoft.Dynamics.EventManagement.CrmPlugins.EventPortalLinkPlugin, Version=1.12.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35
2019-08-20T09:04:13.4788460Z :: Executing PortalCGIApiHandler for route: /api/events/register/
2019-08-20T09:04:13.4788460Z :: Trying to execute controller: events
2019-08-20T09:04:13.4888399Z :: Trying to execute method: register
2019-08-20T09:04:13.4888399Z :: Entered SessionsToAttendeeSessions
2019-08-20T09:04:13.4888399Z :: Found 1 attendees
2019-08-20T09:04:13.4888399Z :: Attendee has 1 sessions, converting to AttendeeSessions list
2019-08-20T09:04:13.4888399Z :: Registering for event with readable event id: SBERO3224291945
2019-08-20T09:04:13.4888399Z :: GetEvent called with readableEventId: SBERO3224291945
2019-08-20T09:04:13.4888399Z :: GetEventByReadableId called with readableId: SBERO3224291945
2019-08-20T09:04:13.4888399Z :: Resolving custom columns for msevtmgt_event
2019-08-20T09:04:13.4968429Z :: GlobalRetrieveMultipleEvents
2019-08-20T09:04:13.5568162Z :: DoesEventEntityContainThePortalPaymentGatewayAttribute: True
2019-08-20T09:04:13.5658098Z :: Found 1 event(s)
2019-08-20T09:04:13.5658098Z :: Event is found: True
2019-08-20T09:04:13.5658098Z :: GetEvent: Event found
2019-08-20T09:04:13.5658098Z :: Event allows anonymous registrations: True Is there a logged in user: True
2019-08-20T09:04:13.5658098Z :: ListPasses called with readableEventId: SBERO3224291945
2019-08-20T09:04:13.5658098Z :: Resolving custom columns for msevtmgt_session
2019-08-20T09:04:13.5738389Z :: Resolving custom columns for msevtmgt_pass
2019-08-20T09:04:13.5918432Z :: 0 passes found
2019-08-20T09:04:13.5918432Z :: Is paid event: False
2019-08-20T09:04:13.5918432Z :: Exists ClientIP: True
2019-08-20T09:04:13.5918432Z :: Exists ClientHost: True
2019-08-20T09:04:13.5918432Z :: Enter verify captcha url
2019-08-20T09:04:13.5918432Z :: Is null or empty clientip: False
2019-08-20T09:04:13.6008183Z :: Using Configuration entity 035368e8-3131-e911-a986-00224800510b.
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_eventmanagementconfigurationid
2019-08-20T09:04:13.6008183Z :: Returned attribute: modifiedon
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_authenticationtokenendpoint
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_configcachecreatedon
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_discoveryendpointurl
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_configcacheexpirationdate
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_configcacheexpirationinsec
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_authenticationresource
2019-08-20T09:04:13.6008183Z :: Returned attribute: msevtmgt_organization_config
2019-08-20T09:04:13.6008183Z :: Assembled url is: 90ef28fdc2b944328997873622297685.svc.dynamics.com:815/.../verify
2019-08-20T09:04:13.6008183Z :: POST request, input URL is [https://90ef28fdc2b944328997873622297685.svc.dynamics.com:815/EventManagement/api/org/5e572b40-f094-4d16-a9cd-0508fe0c262b/captcha/verify?api-version=2016-06-30]
2019-08-20T09:04:13.6008183Z :: Serialized input model is: [{"ClientIp":"212.139.248.177","FlowId":"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","Solution":"64k6mw3","Token":"EUS.e99cd064858041b6b590668919123415","Type":"visual"}]
2019-08-20T09:04:13.6008183Z :: Enter into [RequestToWebApi] function
2019-08-20T09:04:13.6148147Z :: AddAuthentication: starting AcquireToken
2019-08-20T09:04:13.6148147Z :: AddAuthentication: authentication token endpoint is login.microsoftonline.com/72f988bf-86f1-41af-91ab-2d7cd011db47
2019-08-20T09:04:13.6148147Z :: AddAuthentication: authentication resource is microsoft.dynamics.marketing.plugin/
2019-08-20T09:04:13.6498472Z :: The bearer authentication token was successfully obtained.
2019-08-20T09:04:14.4748658Z :: Response received with status: [OK]
2019-08-20T09:04:14.4748658Z :: Exited from RequestToWebApi function
2019-08-20T09:04:14.4748658Z :: Exit verify captcha url
2019-08-20T09:04:14.4748658Z :: Verify captcha value: False
2019-08-20T09:04:14.4748658Z :: Registration failed with error code: Microsoft.Dynamics.EventManagement.CrmPlugins.EventMgmtPlugin.Models.RegistrationResult+ErrorCodes
Thanks,
Swathi