I found the reason for the 500 Server error when backing up to dropbox. If you have a \ (backslash) instead of a / (forward slash) in the directory name for saving to dropbox (the box has the tooltip "The directory within your dropbox account where...") it gives the 500 error.
In my case (my own stupidity) I had a mixture:
/WebsiteBackups\mysitename and changing it to /WebsiteBackups/mysitename fixed it.
It might be an idea for Akeeba Backup to either silently change \ to / when a user types it in, or warn that \ should be /
It is possible there are other characters that should not be allowed in the dropbox folder name, but \ definitely causes the 500 error.
Hope that helps!
Brian
Brian Craigie Wishaw, North Lanarkshire ML2 0PS