If this error occurs during a job run, the quickest way to get the job back into an operational state is to start a new backup chain by forcing the job to create a new Active Full. This will not correct the issue impacting the existing restore points but is advisable to ensure that the job can promptly return to creating new restore points.
As an alternative, if forcing an Active Full is impractical, Veeam Support can assist in investigating whether a repair procedure can be performed to generate and insert the missing summary.xml file manually. However, this procedure is not possible if the issue which caused the summary.xml not to be inserted has also caused the restore point to be corrupted. If the restore point is both missing its summary.xml and is corrupt, the restore point is unrestorable.
If the error occurs during a restore operation, please create a case with Veeam Support. It is possible that using an advanced procedure, Veeam Support may be able to manually generate the missing summary.xml and insert it into the restore point to repair the file. However, most often, the issue that has caused the software not to write the summary.xml into the restore point will have also corrupted the restore point.