Originally Posted by Sabardeyn
I've been trying to recreate this problem, but I'm not getting the error message in a new database. I can still get errors from the original (problem) database.
I know I removed a #customseries_index, but not the #customseries, from a composite column. Thinking back, the books throwing the errors from the original message were books that all had fields filled with sample data - including the now missing #customseries_index.
Thanks for taking the time to try to find a reproducible scenario.
The problem could possibly be related to a bug that I fixed a week ago that caused the series index for custom series columns to occasionally be empty. It would happen if you used bulk edit to set a custom series but not the custom series index. It would display as  but actually be null (None).
To test the theory, try running this search on the original database.
#customseries:true and #customseries_index:false
If this finds books that raise the exception, then the fixed bug is likely the culprit. You should be able to select all the books and use bulk edit to set #customseries_index to a known value then see if the problem goes away.