Yes, you are transferring a call to an extension that is currently forwarding it's calls.
The destination is an extension with no voicemail or a mailbox that is disabled.
Try this: create a brand new extension, and leave everything default. Login to the webclient or mobile client of this extension. Then try to forward a call to it. Your transfer should now succeed, proving transfers in general work ok.
You can then go and look at the other extension you were having trouble with, check what status its currently in, and check what forwarding rules or exceptions it has defined to learn what happens to your original problematic call transfer.
If you still cant find what the cause is, then factory reset the hardware phone and reprovision it. If the problem goes away, then you know that someone performed a call forward on their hardware phone (something we do not support nor recommend you do).