I managed to get sonarr working again 
with the help in this topic: Sonarr fails to start on Synology after latest package update
What I did:
installed this mono package manually: https://archive.synology.com/download/Package/spk/Mono/4.6.2-0096/Mono-armv5-4.6.2-0096.spk
Changed the monopath in service-setup file in /var/packages/nzbdrone/scripts
to MONO_PATH="/var/packages/Mono/target/usr/local/bin"
Then Sonarr works again using the other mono package you installed manually (it has a capital letter M in Mono).
I went straight for v3.x of sonarr and installed this package instead of v2.x from the synocommunity: https://github.com/Safihre/spksrc/releases/download/sonarr-v3-build/nzbdrone_88f6281-6.1_20200413-16.spk
