Hi
Same prob for me and i’ve fix with this 
Remove all mono on ur system, purge/autoremove, etc… and follow this :
- https://download.mono-project.com/sources/mono/mono-4.8.1.0.tar.bz239
- tar xvjf mono-4.8.1.0.tar.bz2
- cd mono-4.8.1
- ./configure
- make
- sudo make install
Find on this topic : [SERVER] Latest Mono 5.0.0 will break the server. Read for Fix