Hmm. I think Linux mint 19.2 is based on Ubuntu 18.04 The packages for that are for Sonic Pi 2.10.0~repack-2.1 which is quite an old version. The same is true for Debian Buster general distribution. The specific Raspberry Pi version has an additional package for a cut down Sonic-Pi 3.1. This version will not support midi or OSC functions. It is not clear what exactly you have installed.
The logs you have are slightly unfamiliar in format, and it looks like you have concatenated the separate files. From what I can see scsynth is launching OK. Usually when the server fails in that case it is because the port allowing communication between the ruby server and scsynth is not availalbe. Later versions make this very clear in the log files.
These are the ports up to version 3.1
Detecting port numbers…
Send port: 4558
Listen port: 4557
- OK
Scsynth port: 4556
- OK
Scsynth send port: 4556
- OK
OSC cues port: 4559
- OK
Erlang port: 4560
- OK
OSC MIDI out port: 4561
- OK
OSC MIDI in port: 4562
- OK
They have changed in later versions. In versions before 3.1 the OSC cues, Erlang and OSC MIDI are not relevant. So check that ports 4556,7 and 8 are available, before starting Sonic Pi. The code which connects to scsysnth is in the file scsynthexternal.rb in the lib/sonicpi folder. Note this folder is in different positions in the source tree depending on the version you are using. It changes for version 3.2dev
Personally, if you are experienced in Linux I would have a go at building the latest 3.2dev version from source. It has much better facilities available in it. There are instructions for building on various versions of Ubuntu referenced in this site (search for ubuntu or mint), and some present in the github distro in the sonic-pi/app/gui./qt folder.
For Buster I have a recipe posted here