OSP 13 to OSP 16.1 upgrade process recommends the usage of ReaR to backup undercloud and control plane. The backup image for the undercloud was generated as instructed but during recovery the filesystems couldn't be mounted.
This was due to a cron task explanation at BZ#1911535. There is a fix for that at BZ#1919174. For tracking the issue on ops13 BZ#1921668
At 16.1.6, by default, the cron job no longer runs a backup at 1:30 in the morning. There is no doc impact.