Thank you both @ahmaddanial and @sven.huester,
I did not know that the usernames had to be identical in the import. I think at least one user changed the name, so I could manually change that in the imported jsonl file.
Users and public channels have already been imported from our first Slack export. I was hoping to now also import the DMs and private channels from the Slack corporate export. Ideally without removing any channels and DMs that have been created in Mattermost in the meantime.
I do not really understand the synchronization statement about the bulk import. I’m not trying to remove information, just add messages and channels.
As I said in my last post, the Id of the violating user changes every time the bulk import tool is executed. I think it is the Id of a new user it tries to create. The given Id appears neither in our mattermost SQL database nor the .jsonl import file.