You do not have the necessary read, write and append privileges on the selected network backup disk

I have OSX Server running on a mac mini in my home. I've been using it for doing Time Machine backups from my MacBook. This worked fine until I changed the password for my user account on the mac mini. At that point, backups on my laptop started failing, indicating that I needed to update the password that the laptop was storing for the mac mini. However, when I try to do so (at System Preferences > Time Machine > Select Backup Disk...), I get the following error:

"You do not have the necessary read, write and append privileges on the selected network backup disk."

The laptop is running OSX 10.9, and the mac mini is running OSX 10.8.

Has anyone using OSX Server for backups experienced something similar to this? Does anyone know how to fix it (without losing all of my previous backups)?

On my iMacs running macOS High Sierra (version 10.13.6) I had to explicitly add filesystem permissions for Time Machine to function correctly over the network. This resolved the You do not have the necessary read, write, and append privileges on the selected network backup disk error message.

I'm using username_A on both the source and destination iMacs.

On the source iMac, I used username_A when configuring Time Machine to run over the network to the shared network drive.

On the destination iMac (the one sharing out the Time Machine backup disk over my home network), I had to add username_A to the list of allowed users in System Preferences → Sharing → File Sharing. I also had to pull up the Get Info window for the shared disk and add username_A to the Sharing & Permissions list at the bottom of the window. Once I had added username_A in both places on the destination iMac, I was able to get Time Machine running over the network.

I still don't know what caused the issue to come up in the first place, but I was able to get around it by going to Server.app > File Sharing > [my backup volume] > Edit Share Point... and then adding my user to the Access section of the page with Read & Write permissions.

I don't know how/why it worked before or why changing the user password caused it to stop working, but doing the above gets around the issue.