Foxmarks: Error: Unknown error 400

From Foxmarks Wiki

If you receive this error during your first synchronization with Foxmarks 2, it may mean that your bookmark file has a recoverable corruption in it. Foxmarks 2 is more strict than Foxmarks 1 about the correctness of your bookmarks; this problem may have been present but dormant in your bookmarks for quite a while.

One likely fix for this problem is to perform an upload from the affected computer to the Foxmarks server. You can do so by going to Tools > Foxmarks > Settings, select the "Advanced" tab, then click the "Upload" button found there. This will overwrite the Foxmarks server with the bookmarks on this computer, though, so make sure that any bookmarks on the Foxmarks server are represented on other computers of yours--you can view your Foxmarks server bookmarks by visiting http://my.foxmarks.com/

Once you've uploaded your bookmarks from one computer, you can then safely merge that upload with the bookmarks on other computers when they next synchronize--if you'd like to initiate a manual sync, you can do so via Tools > Foxmarks > Synchronize Now

Another possible tack for repairing this problem yourself can be found on this page.

Another possibility is that a firewall or anti-malware software is interfering with Foxmarks's ability to contact the synchronization server--we've seen more than one instance of this with Threatfire.