I have the same issue with any android project when I add jCenter dependency on org.apache.ftpserver:ftpserver-core:1.0.6. Project compiles successfully, but it fails when I try to start it.
I realized, that ftpserver-core-1.0.6.jar contains classes from ftplet-api-1.0.6.jar.
To fix the issue, I used local libraries, but deleted ftplet-api-1.0.6.jar. I am not sure that it is totally correct solution, but it works for me.