i have changed my previous answer....sorry for the last incorrect answer.......

the problem for most of you will be because you are running multiple databases.............

when u accept the invitation check the server logs and you'll see the "None werkzeug" in the line where the accept invitation link was received"

the solution is to run the server with --db-filter

eg...     openerp-server --db-filter demobase

when the meeting is accepted , openerp will by default check in demobase

for more clarification check the docs folder in odoo---------odoo/setup/deploy.rst