I would say that either it's a cosmetic bug, there is a non-printing character in the entry (maybe a tab instead of a space?), or there is an IP conflict with a different interface, although that should throw a different error.
18.104.22.168 is a perfectly acceptable address for a /29 of 255.255.255.248.
You can't use 22.214.171.124 (network) or 126.96.36.199 (broadcast) but nothing wrong with what you have.
I see what looks like a fragment of a line. Are you copying and pasting this from a text file to a 9600-baud console port? Perhaps you are overrunning a buffer if so. I'd take it in smaller chunks if so and let the buffer catch up.
Also, try entering the command "ip classless". It should be on by default since around the turn of the century.
You have a bit more than a syntax issue there. The 5660 is basically a switch first and a router secondly. You cannot apply an IP address to a "switchport" so you need to build the sub interface such as below:
interface gigabit-eth 0/1
interface gigabit-eth 0/1.1
ip address 188.8.131.52 255.255.255.248
Now you made it a Layer 3 device and can have an IP assigned to it.That should do it for you! This info is in Adtran's AOS online .pdf documentation as well.