If you have two duplicated contacts, A and B, and contact B has an associated event registration. When you try to merge the two contacts and make A the master, the merge fails with the message "We can't process your submission because it includes new values for one or more read-only fields."

The trace log shows some more details:

ErrorCode: 0x80040265
Message: We can't process your submission because it includes new values for one or more read-only fields.;
[Microsoft.Dynamics.EventManagement.CrmPlugins.EventMgmtPlugin: Microsoft.Dynamics.EventManagement.CrmPlugins.EventMgmtPlugin.Plugins.PreventModificationsOfReadOnlyFieldsPlugin]
[fe57a9f6-98a7-e911-b823-00155dc24900: Microsoft.Dynamics.EventManagement.CrmPlugins.EventMgmtPlugin.Plugins.PreventModificationsOfReadOnlyFieldsPlugin: Update of msevtmgt_eventregistration]

2019-11-19T16:36:44.5129350Z :: PreventModificationsOfReadOnlyFieldsStrategy for 0
2019-11-19T16:36:44.5129350Z :: This action is restricted. At least one field that is being modified is marked as 'read-only' and therefore is not allowed to update the entity.
2019-11-19T16:36:44.5129350Z :: Begin: GetLocalizedMessage, messageCode: ReadOnlyFieldsCannotBeModified
2019-11-19T16:36:44.5285575Z :: User language is: 1033
2019-11-19T16:36:44.5285575Z :: Begin: RetrieveJsonWebResourceByName, l...).

The plugin should ideally cope with the situation where the contact field is being updated due to a merge happening and allow the field to be updated, as otherwise there is no way to merge these duplicated records.