We have the same issues after updating, the update itself was fine and it seems like only the alerts are broken.
Since we use observium to monitor our infrastructure and network this is a pretty bad thing, luckily we've got nagios to monitor our server-infrastructure so we're not totally in the dark.
This is the output from alerter.php:
mon2:/opt/observium# ./alerter.php -h host.domain.tld -dd
DEBUG!
DEFINITIONS loaded by: 0.0401499 ms
DEFINITIONS in memory: 15.9MB
SQL[PHP Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE) in /opt/observium/libs/Doctrine/SqlFormatter/SqlFormatter.php on line 36
Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE) in /opt/observium/libs/Doctrine/SqlFormatter/SqlFormatter.php on line 36
After upgrading the distro and then to php 7.3 the alert module works again.