ORA-19815, ORA-01031 and ORA-12514 error
The last line in alert.log says that your instance should have been killed (Shutdown abort). This is why you can't connect: listener is up and running and responds to tnsping, BUT the instance is not available (listener does not currently know of service requested in connect descriptor").

You can verify with:  

ps -ef | grep pmon  


and check if the process ora_pmon_INSTANCE is present or not.

First you have to free space in archive destination, moving elsewhere archived logs, change parameter db_recovery_file_dest_size to a bigger size and/or add disk space to filesystem where path /u01/app/oracle/fast_recovery_area/XE/archivelog/2018_12_01 resides.

Then restart the instance. Sometimes I saw redolog corruption after this scenario, but this is a different issue, you can face of after resolving the first one (startup the instance).