The backup destination folder is created automatically and titled according to how the user names the backup task. In order to differentiate between different backups, the subfolders are named with dates and times indicating the time of the backup. A corresponding folder is then created like in the picture below:
If you use copy-and-paste to back up, the part of the path circled in red won’t be created as part of the file name. So even if the file name is very close to the length limit, you can still back up files by coping and pasting.
The actual length of the file name includes drive letter, colon, backslash, name components separated by backslashes, and a terminating null character. Long file names or files placed in the bottom layer folders might exceed the maximum name length supported by the system, resulting in backup task failure.
Please refer to the link below for more details:
We suggest that you rename files that have file names too long or back it up manually to avoid backup failure.