I have this exact problem every now and then, we're at 6.5.1.
The collection name is nothing peculiar and can be as trivial as 'Item Data' or 'Customer Data'. In none of the cases did the collection contain a nested collection, they were either undefined or defined with simple columns. As the error in my case occurs while I'm developing something new in an existing process, it does not yet contain data, but has obviously already been defined in the process.
As a remedy, I've noticed that clicking the required collection in the list and waiting a few minutes usually prompts the process to accept my choice. In other cases I closed the process, reopened it after which the problem did no longer occur.
I have not tried deleting the loop or collection and recreate them to see if this worked better. I'll keep in mind to try that next time.
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Happy coding!
Paul
Sweden
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Happy coding!
Paul, Sweden
(By all means, do not mark this as the best answer!)