It looks like all the entries are associated with a Failed Transactions from Event Espresso.
To confirm, you are looking in the REST API Log? Not the server’s error logs, correct?
I don’t believe you should have any entries in the REST API log for EE transactions.
Maybe it is another problem but some of my client had this error message: “This registration step could not be completed. Please refresh the page and try again.”
Do I have to open a new post for this problem ?
That’s a general error and means something went wrong during the registration, if you look in the servers error logs (not the REST API Log), can you see any errors from EE there?