ResolutionThere are multiple potential solutions to this issue, depending on the version of Tableau Server.
Note that Steps #3 and #4 on the below list will also resolve a number of potential issues where the backup itself is failing, which can lead to this situation.
Step 1: If no backup job is currently running, it is safe to manually delete these files.
Step 2: For Tableau Server 2018.2, 2018.3, or 2019.1, upgrade to at least 2018.2.7, 2018.3.6, 2019.1.3 to ensure that you are not experiencing a known issue (ID #866789) resolved in the April 2019 releases of Tableau Server.
Step 3: Confirm that the file/folder permissions for the Tableau Server install and data directory location(s) are not preventing Tableau Server from normal access to move and delete temporary files.
Step 4: Confirm that no antivirus, monitoring, disk backup, or other security tools are locking the files and preventing Tableau Server from moving or deleting the files.