Thanks you for reporting this issue, it's the first I have heard of Wysija Newsletter. I downloaded Wysija Newsletter but I cannot recreate the 500 error you are getting. I would like to fix any conflicts between my plugin and any other plugins.
Can you please contact me directly so that I can work with you on finding a solution to this problem?
My direct email is: eli At gotmls DOT net
Aloha, Eli