This might be caused by the resequencing order of your sheet numbers, whereby a sheet which is yet to be renumbered owns a number which you are attempting to use to renumber another sheet, hence its read-only as you haven’t ‘freed’ it up yet.
If that is the case use BimorphNodes Sheet.RenumberRename as it’s built to circumvent this issue.
There’s some more info about this phenomenon here:
I wouldn’t think it is a sequencing issue since he is merely appending things to the end of the sheet numbers. Therefore I don’t think he would have any duplicates. The “null” in the middle of the output is interesting, but appears to align with this issue.
Still seems like using a different parameter route or the node you have created would be a good way to go.