Cron errors / scheduled posts not working

I’ve suddenly started getting the following error at the top of each page of my site (front- and back-end):

Warning: Cannot unset offset in a non-array variable in /*****/wp-includes/cron.php on line 151
Warning: Cannot unset offset in a non-array variable in /*****/wp-includes/cron.php on line 153

In addition, scheduled posts are no longer posting, which I think may be related to the same issue. I’ve tried all kinds of fixes for this (editing the timeout time in cron.php, creating a new cron job to replace WP’s, etc.) and nothing makes any difference. I’ve tried deactivating all the plugins I’ve update recently but it doesn’t help, nor does replacing all the core WP files. I can’t figure out what could have caused this or how to fix it.

Thank you!