The workaround is as described in the comments of that bug. (Edit the .bzr/branch/branch.conf files for those branches via SFTP.)
Bzr itself could be more helpful in detecting this situation (a loop in stacked-on locations) and guiding the user towards fixing it, so I'll leave this bug open for that aspect.
The root cause of this is bug 377519: lp:~openerp/openobject-server/5.0 says it is stacked on lp:~openerp/openobject-server/trunk, but lp:~openerp/openobject-server/trunk says it is stacked on lp:~openerp/openobject-server/5.0. This is almost certainly due to branch renames done via Launchpad, but due to bug 377519 Launchpad doesn't update the stacked-on locations when renames happen.
The workaround is as described in the comments of that bug. (Edit the .bzr/branch/ branch. conf files for those branches via SFTP.)
Bzr itself could be more helpful in detecting this situation (a loop in stacked-on locations) and guiding the user towards fixing it, so I'll leave this bug open for that aspect.