Usually that happens with large files (virtual machine disks). Sometimes transfer gets blocked just after a few megabytes is copied, sometimes it takes longer to get blocked.
Maybe that’s not the right forum, but I am using Ubiquiti UDMP.
If I set it to not block and ignore those alerts, I would not get one if there is a real threat, Am I right?
Sorry, I know nothing about this.
I would suggest using copy/sync tools such as Teracopy or Gs Richcopy 360 to handle your backups to Synology NAS and securely transfer your data without any problems/errors/alerts