This is normally an error reported in connection with Scheduled Tasks (i.e. cron jobs), although it's not impossible I suppose.
Can you do three things...
- enable Debugging by copying the lines from section 7 of config-dist.php to config.php
- completely delete the 'cache/' directory under your moodledata / $CFG->dataroot directory (not your Moodle program code)
- Purge the caches using https://docs.moodle.org/35/en/Administration_via_command_line#Purge_caches
If you're lucky it might fix it. Failing that we should at least find where the error is originating.