These errors should not prevent OJS from running, but in my case the log files became enormous (several gigabytes). The fix is to add two additional classes of PHP errors in /lib/pkp/classes/core/PKPApplication.inc.php, after line 52:
if (defined('E_WARNING')) $errorReportingLevel &= ~E_WARNING;
if (defined('E_NOTICE')) $errorReportingLevel &= ~E_NOTICE;
See the solution by @Jeffery_Triggs in post PHP 7 warnings filling up error log after upgrade to 2.4.8.4 for more info.
Cheers, Ales