An Oracle server that had no maintenance for way too long, would not start the database anymore. Trying to start up the database gave an "ORA-03113: end-of-file on communication channel" error.
Some Googling led me to https://dba.stackexchange.com/questions/49682/ora-03113-end-of-file-on-communication-channel-on-startup. It described my problem and the solution(s) exactly, so I thought I'd share it here.

The output of the adrci "show alert" command looked like this in my case: Errors in file /u01/app/oracle/diag/rdbms/isrp/ISRP/trace/ISRP_arc2_12405.trc: ORA-19809: limit exceeded for recovery files ORA-19804: cannot reclaim 41501696 bytes disk space from 107374182400 limit ARC2: Error 19809 Creating archive log file to '/u03/flash_recovery_area/ISRP/archivelog/2019_02_11/o1_mf_1_19627_%u_.arc' 2019-02-11 16:26:04.722000 +01:00 ARC3: Archival started ARC0: STARTING ARCH PROCESSES COMPLETE Errors in file /u01/app/oracle/diag/rdbms/isrp/ISRP/trace/ISRP_ora_12399.trc: ORA-19815: WARNING: db_recovery_file_dest_size of 107374182400 bytes is 99.99% used, and has 11255808 remaining bytes available.