There is some arbitrary length limit on root passwords that isn't disclosed anywhere (or reported if you exceed it.) I had to truncate my password several times to get it to actually work.
4cX|%:8emq94L8X
,6r7oh97S|b"+1Z
2#E24IC^&G(x47P
I tried an alphanumeric of the same length (15 characters) and it worked. So i doubt it's a length issue.