Well, I must admit I didn't really try the build like you did.  I just
did a gvimdiff of roundup.pot and de.po.

Concerning the 2nd problem, it looks like there is a constraint which
requires the msgstr to begin with a '\n' if the msgid does.  If you
insert a new line 2266 with "\n", and do the same in line 2129, it
should work.

(or at least the error you reported should go away ;-)