i recently bought the mac mini 2018 and transferred my data from a mac mini 2014 via airport time capsule.
when i opened Time Capsule it asked me if i wanted to 'inherit' from my time capsule[ 3TB 'standup model} and thus i did.
Did you remove the old 2014 Mac from the network?
Did you change the share name of the 2018 Mac? That might help.
Inheriting a back is a really bad idea.
It is very easy to destroy the existing backup. And fail to produce reliable backups over the top.
Since you have already inherited the backup you can try if you want to continue by doing a verify.. hold down the Option key while you click the Time Machine icon in the computer.. the option to verify will appear.. run it and see if it can finish. If so try again to start a backup. If it stalls or fails.. I would wipe the Time Capsule and start a new backup. Do you still have the 2014 Mac Mini.. and is it still working with all your files? If so just wipe the TC .. a quick erase is fine.. and make a new backup from both the old and the new computer.
If you want to keep the existing backup.. archive it to a USB drive before you wipe the TC drive. Archive and Erase are both available on the disk tab of the TC in airport utility.
