Hi @Snipper, welcome to the forum!
The “Error reported while accessing file” warnings are likely caused by you running Duplicati as a normal user then having it try to back up system parts of the hard drive. You can get around this by either running it as an admin (such as by running Duplicati as a service) or specifically excluding folders to which Duplicati doesn’t have access rights.
The “Failed to process file” error seems to be related to the Duplicati step where it tests some of your backup files to make sure they’re still valid. Unfortunately, I can’t tell whether the issue is related to fetching the file or using it after it’s been fetched.
(By the way, I edited your post to make the messages easier to read.)