The following MWE reproduces the issue
The problem is a missing
basque.ldf. Currently (v1.0f, 2005/03/29), ll. 90-91 read
but that should be
% after the opening curly brace (see What is the use of percent signs (%) at the end of lines? (Why is my macro creating extra space?) for details about
% at end of lines).
Contact the maintainer of
babel-basque about this small oversight. If you want a short MWE for the bug report, you can use
If you need a solution now, you could create a local copy of
basque.ldf and fix the issue there – or you could add
to your preamble. The
\unskip counters the space that was introduced by the missing
%, but this is only required and desirable in horizontal mode (in vertical mode the
\unskip would get rid of vertical space that should probably remain as is).