Plain Stupid

[Qlumsee]

The game has just told me I can't access a raid area because I'm in a raid group.

Seriously, get your house in order before you make new content.

[SoulDustar]

I am going to take a shot in the dark here and say you were trying to do Tower of Despair

https://ddowiki.com/page/Tower_of_Despair/

Characters who are entering this raid for the first time may need to run their characters across the Devil Battlefield to the Tower Cave, the raid entrance. Since you cannot enter the Battlefield while in a raid party, it will be necessary to leave your group for the journey to the Tower Cave. A resourceful party member may also be able to teleport you there. A Bracelet of Friends works here for example.

Once in the Tower Cave, speak to the Yugoloth scout there. He will add a second option to the Amrath portal, allowing you to teleport directly to the Tower Cave for future runs.

[Qlumsee]

No, Project Nemesis. This game is more screwed than some think.

[Arkat]

The game has just told me I can't access a raid area because I'm in a raid group.

Was your raid group (or some folks in it) in a raid other than PN?

[AlcoArgo]

Never had an issue getting into PN. Screenshot or it didn't happen.

[Halciet]

On Hardcore our group had to split up to run Hiding in Plain Sight because it would not let some of us in while in the same party. Dropping party immediately made it work. Wish I had screenshot the error message.