This problem still happens with rev 71477 (final 6.1 release).
Reopening report.

Note that I also cannot login using a valid username and password on
Solaris SPARC 32-bit (username "prestoat2000", you can find the
password) though I was able to login on Solaris 10 on x86 and
successfully submitted a test bug report on that platform.  We have
an older version of libcurl (not sure what version, but some of the
files are dated from 2004).  Don't know if this might be the cause
of the login failure problem.  I can find out the libcurl version if that
would be helpful.