Unrelated to the PSN maintenance scheduled for today, many PS4 owners have reported problems with their PS Plus membership.
Several PS4 owners have experienced issues of online games, being locked out of online play in games like Killzone: Shadow Fall, FIFA 14, Call of Duty: Ghosts, and Battlefield 4. Sony's English community manager for Europe Chris Owen confirmed on Twitter that this is a problem on Sony's end, and Sony is currently investigating.
The error tells users they cannot access the content because they do not have a PlayStation Plus subscription. The reports come from those who indeed have a PS Plus membership. Today's scheduled PSN maintenance affects online play for users who have not signed into PSN within the past five days, but the official Killzone Twitter account stated the online gaming issue is "an unexpected event, confirmed by Sony" and thus not related to PSN maintenance, which has just begun.
Assuming the problem is fixed within the five-hour maintenance window, once it's fixed, all users who have recently signed into PSN can get back to playing their games online. Beyond problems with the network not recognizing PS Plus memberships, the network seems to have gone down early for some.
Owen is unsure how long the fix will take, but he will post to Twitter once the problem is resolved. For now, if you're one of the affected users, rest assured that your PS Plus membership is fine, and Sony will hopefully have the situation under control soon.