by Ken Task.
Ahhhh ...... issue could be running 2.9.x under PHP 7.x
See:
https://docs.moodle.org/dev/Moodle_and_PHP7
Can verify that a 2.9 doesn't work with 7.0.x on Linux ... I had several sandbox sites on one server and decided to upgrade php to highest 7.0.x for versions 3.1/3.2 sandboxes. Broke the 2.9. No problem, however, the 2.9 site was installed via git and it was easy to fix by upgrading to 3.0.highest at that time via git.
You might double sheck that hotpot and taskchain are comapat with 3.0 and beyond.
https://moodle.org/plugins/pluginversions.php?plugin=mod_hotpot
https://moodle.org/plugins/pluginversions.php?plugin=mod_taskchain
Think I'd upgrade to 3.0 first and the get the site running there first ... with no errors in backups, etc.
Backup what you have before upgrading .... code directory backup and DB backup minimal. The moodleata directory shouldn't be changing.
'spirit of sharing', Ken