Time Machine Server - one MBP can back up, another can't. Why? So I have Server 5.2 running on a Mac mini, running OS X 10.11.6.

I have two MacBook Pros trying to use it as a Time Machine server; the backup disk is offered to both MBPs when setting up Time Machine. I select Encrypted backups on both MBPs, and use the same credentials to connect to the disk on both, then enter encryption passwords for both.

Time Machine is set up successfully on both MBPs. They also both have SMB and AFP enabled under File Sharing.

One MBP backs up using Time Machine just fine, the other says that the backup fails because the network username and password are wrong. It suggests re-adding the backup disk. I've done this countless times, and triple-checked the credentials used on the second MBP, but I can't get the backup to work.

Any suggestions to advice? Thank you!
So, the problem was that the second MBP needed the Time Machine server credentials User in all lower case. The Mac mini has a local user, let's say Arthur. I configured the second MBP to use the credentials Arthur/(a password) and the connection wouldn't work. Configuring it as arthur/(a password) worked just fine.

Thanks to Allan I found this by mounting two volumes from the Mac mini, one using SMB and one using AFP, and noticed that the AFP mount showed the user as 'arthur', and the SMB showed the user as 'Arthur', even though I'd logged in as Arthur for both mounts.