I had the same issue with XFCE. It would try to relaunch it at startup and fail with that error. The cause of it is that the Enpass and EnpassHelper programs register themselves with the session manager, which caused the DE to try and relaunch them. The easiest way I've found of fixing it is to add this line to the runenpass.sh script:
Add it just before the 'LD_LIBRARY_PATH' line. This prevents Enpass from finding the session manager, and allows my manually added autostart to work.