Paul thinks it should remain a String but what is missing is how to encode the bytestring as a string for display.

But why should the server convert the value to be displayed. It's up to the client to display the value or, even better, display the corresponding event. If the client wants to match the EventIds the string needs to be converted back into a bytestring. In this case, there are two useless conversions.

More arguments from Adrian for changing this to a ByteString:

• Because of the missing conversion rule no one could have implemented this according to the specification
• A major advantage of OPC UA vs. classic OPC on the server side is the removed requirement for type conversions, which would be introduced by this again
• Why does the server need to bother with the conversion from ByteString to String, when its only used for display purposes

Agreed it needs to be changed to a ByteString. The assumption is this breaking change will not affect interop of shipping product. Needs new 1.04 Errata to fix the 1.04.10 Errata entry.

Changed DataType of CertificateErrorEventId property from String to ByteString in the 1.05.03 draft.
Added entry to 1.04.12 Errata.

Agreed to Errata edited in Web meeting.

1.04.12 nodeset was not updated to reflect the fix.