Good suggestion! I found /var/log/roundcube and in it I found an errors file with:
[06-Jan-2019 13:36:48 -0600]: DB Error: Configuration error. Unsupported database driver: in /usr/share/roundcube/program/lib/Roundcube/rcube_db.php on line 84 (GET /index.php)
I then checked /var/log/apache2/error.log and found:
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/apcu.so' - /usr/lib/php/20151012/apcu.so: cannot open shared object file: No such file or directory in Unknown on line 0
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/imagick.so' - /usr/lib/php/20151012/imagick.so: cannot open shared object file: No such file or directory in Unknown on line 0
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/memcache.so' - /usr/lib/php/20151012/memcache.so: cannot open shared object file: No such file or directory in Unknown on line 0
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/ssh2.so' - /usr/lib/php/20151012/ssh2.so: cannot open shared object file: No such file or directory in Unknown on line 0
[Sun Jan 06 00:06:14.077278 2019] [mpm_prefork:notice] [pid 1948] AH00163: Apache/2.4.29 (Ubuntu) OpenSSL/1.1.0g configured -- resuming normal operations
[Sun Jan 06 00:06:14.077289 2019] [core:notice] [pid 1948] AH00094: Command line: '/usr/sbin/apache2'
With info like this, a newbie like me could try to debug, however, I'm hoping someone more experienced could debug more quickly. Does the reference to "/usr/lib/php/20151012" imply a PHP 5 vs. 7 issue?