Do you have any other plans that successfully back up to that destination?
It's unlikely that this is the only error in the logs, so for troubleshooting purposes try re-generating creds for that destination and if that doesn't help send us the diagnostic logs from the machine. You can do that via Remote Management tab by clicking on the gear button and selecting "send logs".

Hi all, we have a piece of information to share. In our case, for the same error message logged, we exclude the folder that generates the constant failures, thats showing always the same files in the failure report log. And when running the cloud backup task, its finished sucessfully. So we will rename the warned files, removing the special characters in the names of each one.

We post the task results, after renaming the logged files.