Bug#623039: incompatible with utf8x inputenc
The biblatex documentation is quite clear about the incompatibility:
| 1.5.4 Incompatible Packages
| ucs The ucs package provides support for UTF-8 encoded input.
| Either use inputenc's standard utf8 module or a Unicode enabled
| engine such as XeTeX or LuaTeX instead.
Also, according to the upstream author, biblatex has never been
compatible with ucs. The error message was just added recently. See
David Paleino wrote:
> This should at least be documented in NEWS.Debian, since it's a big
> change from the version in testing.