Heyo Folks, teams been looking into this and it is a login check firing off in the wrong order.
When you first login during Franks Free chests you would have gotten a Pirates Plunder. If you had a full bag it would have given this message and asked you to relog, upon relog you would have gotten the bag.
Now the issue comes if you already gotten a Pirate Plunder and relogged, you could see this message again if you had a full inventory even though there isn't a second Pirate Plunder waiting to be collected.
This was caused by the login flags being in the wrong order, it was supposed to check if your eligible for a Pirate Plunder and then check if you have inventory space. It is doing it the other way around atm so if you have a full bag even you will see this message even if you collected the Free Pirate Plunder already.
The team is working on clearing up these messages.