No luck, I can log in sourceforge with this password and still have this result:
monnier@...1898... /cygdrive/d/src/inkscape/inkscape
$ svn ci src/widgets/toolbox.cpp share/icons/icons.svg
Authentication realm: <
https://inkscape.svn.sourceforge.net:443> SourceForge Subversion area
Password for 'monnier':
Authentication realm: <
https://inkscape.svn.sourceforge.net:443> SourceForge Subversion area
Username: o__b
Password for 'o__b':
svn: Commit failed (details follow):
svn: MKACTIVITY of
'/svnroot/inkscape/!svn/act/96057da8-f114-11dc-a774-f13b993f4ba9': 403
Forbidden (
https://inkscape.svn.sourceforge.net)
svn: Your commit message was left in a temporary file:
svn: '/cygdrive/d/src/inkscape/inkscape/svn-commit.5.tmp'
I guess this has some relation with being behind a proxy. I'll try from home to see if it's better.