I am having the same exact issue on Fedora 31 GNOME 3. When I do java -jar TechnicLauncher.jar in terminal it goes
Portable settings dir: /home/nt1036/.technic/technic
Setting up logging
and then exits... It puts the same exceptions into the log file as OP. Interestingly if I run it on Java 11, it does the same thing but prints out the exceptions instead of putting them in the logs.
I really need some help with this. I tried it on Debian and it still wasn't working.