The quickest way to troubleshoot plug-in conflicts is to disable all of your plug-ins and then try testing the functionality that is broken. If it works, start activating plug-ins a few at a time and then retest the upload. It should be relatively easy to the activate all plug-ins and then reactivate them without having to remember which ones you deactivated. WordPress will create a recently active list of plug-ins so you don't need to guess which ones to reactivate. If for some reason after deactivating all of the plug-ins you still have the same issue, I would recommend very quickly activating one of the default WordPress themes such as twenty eleven and then retesting. So far though, in nine out of 10 instances it is a plug-in that causes the problem.
If you are unable to narrow it down to a particular plug-in or you think you need further help from us, go to the help tab within SendPress on your site (yourURL.com/wp-admin/admin.php?page=sp-help). On the help tab there are two sections. The first is called Debug Information and the second is called support info. If you are SendPress Pro user you can copy and paste both sections into a new support ticket to get assistance. If you are using the free version, start a new topic in the WordPress forums and someone will do their best to help you figure out the problem.