Some of the bugs fixed in 1.5.5 include:

    Fix an issue where logging in via Facebook would immediately log you out again
    Prevent a database error when using the IP Board 4.0/4.1 importer with a IPB 4.1 database
    Resolve a timeout when dealing with long data URLs
    No longer require a Facebook App ID to be configured to display Open Graph metadata
    Workaround for a PHP 7.0.0 / 7.0.1 bug (fixed in 7.0.2) which causes a "Invalid numeric literal" error to be thrown on some [PHP]
    Fix for "Undefined index: associate_account" error
    Fix for a potential race condition involving G+ button if it hasn't loaded (or has been blocked)
    Fix for an "array to string conversion" error in fb_channel.php
    Workaround for an issue with pasted content being distorted when pasting from Microsoft Edge
    Fix for an issue which clears custom field values from a form after returning to it after an error (such as registration)
    Implement retrieving saved draft messages from the "Reply to Thread" page
    Implement an option to prevent sending mass messages (conversations) via the control panel to users that don't want to receive emails from the admin
    Force rebuild of rebuilding the add-ons cache after changing an add-ons addon_id
    More consistent ordering of a users's IP addresses in the control panel
    Skip email_bounce and email_confirm(_edit) states if a user has just reset their password
    Include thread prefix in subject of thread watch emails
    Resolve an inconsistency with use of ISO-8601 weeks and years causing a display glitch in the Weekly Statistics chart