webapp-config 1.50.15 fails to correctly interpret any symbolic permission string which sets no permissions for one of the categories. For example it claims (after fixing the typo in the error handling, see bug #133530) that "u=rw,g=r,o=" is invalid when it should interpret it as octal mode 0640. The problem is the regular expression which each part is matched against in WebappConfig/permissions.py which requires between one and three of r, w or x to appear when it should be requiring between zero and three. Line 54 of that file should change from: valid = re.compile('^([ugoa]{1,4})([+-=])([rwx]{1,3})$') to valid = re.compile('^([ugoa]{1,4})([+-=])([rwx]{0,3})$')
as a workaround you can use o-rwx
Fixed in webapp-config-1.50.16. This version is available from my overlay (http://overlays.gentoo.org/dev/wrobel) and will be moved to portage next week. Thanks for the report and the fix!
Marked fixed.