I have the same issue (Unable to perform off-host backup for <hyper-v host name> because storage rescan did not complete for this host yet).
Kindly note that this VM was moved to another volume that is mapped to the same Hyper-V host.
We have Veeam v9.5 installed. The job is a replication job.
We tried doing a Rescan of the Hyper-V Host (Right click the host > Rescan), same warning in the replication job.
We tried re-adding the Hyper-V Host (Right click the host > Properties > Next (following the wizard) > ... > Finish), same warning in the replication job.
We tried removing the VM from the replication job > saving the job > then re-adding the VM to the job again > saving the job, same warning in the replication job.
We then tried to create a new replication job containing this VM, same warning in the replication job.
The job is giving the above warning however, it is re-assigning backup infrastructure resources and continuing with replication. So the replication is succeeding but still generates this warning. Also when it is re-assigning backup infrastructure resources, it is assigning the same Hyper-V Host as backup proxy (as detailed in the image found using this link: https://we.tl/m5kfmFwMec
Thank you in advance