In a recent journey history, I see many records for which processing failed, see image below). However the status is quite vague "Failed - One or more errors occurred." How can I get more information on what failed exactly or debug these failures inside Marketing Cloud?
You will not have the tools to debug these, this is something only the Salesforce Marketing Cloud support will be able to check.
If you were actually using Marketing Cloud Connect (as you have tagged this question) then you could turn on logging for the Connector to see detailed log for Contact injection process from SF to SFMC. However based on the ID's they are clearly not SF records.
In case you don't want to wait for the support, you can also detail your question with all the relevant information about this Journey, like:
- What is the Entry criteria for this Journey
- What is the step the failure happens
- How is your target DE built (attributes/types)
- How are the Contacts injected to the Journey
This could only be analysed with the help of Salesforce support. It turned out that there was a SocketTimeoutException thrown at Marketing Cloud end (we saw it in the MC logs).
The reason for the SocketTimeout Exception remained unclear, the messages certainly did not reach the target server (we checked firewall logs for TCP sessions established and SYN.ACK on load balancer). We assume it must be a routing problem of some kind that is only triggered on a high volume (>50k of campaign triggers).
As a workaround we switched from on-premise to Heroku for hosting the app and the problem resolved.
P.S.: In the heroku setup, we found duplicate sendings of campaign triggers and cut off message streams in random seldom cases. Both were not shown as failed in above Journey History.