myselfThanks for your answer but I did not add the addons directory (the one in /usr/lib/python2.7/dist-packages/openerp/addons)  myself, it's the original location with all addons.

What I wanted to do was adding an extra folder where I could put the extra and custom addons so they're not mixed with the 280 other ones. 

I read somewhere that it's possible so I tried adding a 2nd folder but it looks like I'm missing something...

I tried removing the /usr/lib/python2.7/dist-packages/openerp/addons and leave only /opt/odoo/addons and it's still not working, I got more errors:

"Could not get content for /web/static/lib/cleditor/jquery.cleditor.css defined in bundle 'web.assets_backend'.\A 
Could not get content for /web/static/lib/jquery.textext/jquery.textext.css defined in bundle 'web.assets_backend'.\A 
Could not get content for /web/static/lib/select2/select2.css defined in bundle 'web.assets_backend'.\A 
Could not get content for /web/static/lib/bootstrap/css/bootstrap.css defined in bundle 'web.assets_backend'.\A 
Could not get content for /web/static/lib/jquery.ui.bootstrap/css/custom-theme/jquery-ui-1.9.0.custom.css defined in bundle 'web.assets_backend'.\A Could not get content for /web/static/lib/jquery.ui.timepicker/css/jquery-ui-timepicker-addon.css defined in bundle 'web.assets_backend'.\A Could not get content for /web/static/lib/jquery.ui.notify/css/ui.notify.css defined in bundle 'web.assets_backend'.\A Could not get content for /web/static/src/css/base.css defined in bundle 'web.assets_backend'.\A Could not get content for /web/static/src/css/data_export.css defined in bundle 'web.assets_backend'.\A Could not get content for /base/static/src/css/modules.css defined in bundle 'web.assets_backend'.\A Could not get content for /account/static/src/css/account_move_reconciliation.css defined in bundle 'web.assets_backend'.\A Could not get content for /account/static/src/css/account_move_line_quickadd.css defined in bundle 'web.assets_backend'.\A Could not get content for /account/static/src/css/account_bank_and_cash.css defined in bundle 'web.assets_backend'.\A Could not get content for /account/static/src/css/account_bank_statement_reconciliation.css defined in bundle 'web.assets_backend'.\A Could not get content for /base_import/static/src/css/import.css defined in bundle 'web.assets_backend'.\A Could not get content for /base_setup/static/src/css/base_setup.css defined in bundle 'web.assets_backend'.\A Could not get content for /board/static/src/css/dashboard.css defined in bundle 'web.assets_backend'.\A Could not get content for /calendar/static/src/css/calendar.css defined in bundle 'web.assets_backend'.\A Could not get content for /hr/static/src/css/hr.css defined in bundle 'web.assets_backend'.\A Could not get content for /im_chat/static/src/css/im_common.css defined in bundle 'web.assets_backend'.\A Could not get content for /im_chat/static/src/css/im_chat.css defined in bundle 'web.assets_backend'.\A Could not get content for /mail/static/src/css/mail.css defined in bundle 'web.assets_backend'.\A Could not get content for /mail/static/src/css/mail_group.css defined in bundle 'web.assets_backend'.\A Could not get content for /mail/static/src/css/announcement.css defined in bundle 'web.assets_backend'.\A Could not get content for /portal/static/src/css/portal.css defined in bundle 'web.assets_backend'.\A Could not get content for /sales_team/static/src/css/sales_team.css defined in bundle 'web.assets_backend'.\A Could not get content for /share/static/src/css/share.css defined in bundle 'web.assets_backend'.\A Could not get content for /stock/static/src/css/stock.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_calendar/static/src/css/web_fullcalendar.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_calendar/static/lib/fullcalendar/css/fullcalendar.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_diagram/static/src/css/base_diagram.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_gantt/static/src/css/gantt.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_gantt/static/lib/dhtmlxGantt/codebase/dhtmlxgantt.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_graph/static/src/css/graph.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_graph/static/src/css/nv.d3.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_kanban/static/src/css/kanban.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_tests/static/src/css/web_tests.css defined in bundle 'web.assets_backend'.\A Could not get content for /web_view_editor/static/src/css/view_editor.css defined in bundle 'web.assets_backend'."