The issue: when viewing a banner you get a fatal error / white screen of death or this text: There has been a critical error on your website. Please check your site admin email inbox for instructions.
Solution: If you have lots of users on your WordPress site you may need to adjust the advertiser role (General Settings > Placement Settings > Filters > Advertiser Role). By default this is set to “subscriber” and this will load all users in the advertiser section. When there are too many users this could result into a fatal Out of memory error.