Cron job error when Talk is activated

Cron job is running fine, however when Talk is activated we get this error (
matterbridge is not installed).

Error: Call to undefined function OCA\Talk\proc_open() in /…/apps/spreed/lib/MatterbridgeManager.php:833
Stack trace:
#0 /…/apps/spreed/lib/MatterbridgeManager.php(737): OCA\Talk\MatterbridgeManager->runCommand()
#1 /…/apps/spreed/lib/MatterbridgeManager.php(863): OCA\Talk\MatterbridgeManager->killZombieBridges()
#2 /…/apps/spreed/lib/BackgroundJob/CheckMatterbridges.php(68): OCA\Talk\MatterbridgeManager->stopAllBridges()
#3 /…/lib/public/BackgroundJob/Job.php(79): OCA\Talk\BackgroundJob\CheckMatterbridges->run()
#4 /…/lib/public/BackgroundJob/TimedJob.php(63): OCP\BackgroundJob\Job->execute()
#5 /…/cloud/cron.php(128): OCP\BackgroundJob\TimedJob->execute()
#6 {main}

Doesn’t anyone know? Cron job is set to run every 5 minutes according to the manual. We get this error once or twice a day even if Talk is not being used, usually about the same time 11:30 am and 11:30pm.