Error while installing CiviVolunteer on civicrm 4.6.5
Based on this screenshot, two possibilities occur to me, neither of which I like very much:


Something is different about 4.6.5, and it no longer accepts 'Multi-select' as an HTML type. In the code for the extension, this appears on line 433 of CRM/Volunteer/Upgrader.php. In some other usages of this HTML type, I see it referred to as 'Multi-Select' (note the different capitalization) -- it could be that capitalizing the S solves the problem for you. If so, please let me know, and we'll make sure that goes into the next release.
I see you're not running CiviCRM in English. Is your site running in French only, or is it a multilingual site? I'm not sure yet why this would matter, but I've seen strange things occur with the schema changes, etc. that come with a multilingual site -- there are some circumstances CiviVolunteer doesn't account for. It seems like a long shot that the problem is related, but I thought I'd ask anyway.


The failure occurs during a step of the installation that creates custom fields for the volunteer contact subtype. Since the API call that performs this task has worked with previous versions of CiviCRM, I suspect something has changed with the parameter validation, either in the API itself or further down the stack. Either this change was intentional and CiviVolunteer should be updated, or it was unintentional and we should consider filing a regression bug.

Note: I haven't yet tried to reproduce this on a test copy. I'd say this is a necessary step before speculating much further.