Today I was just doing some tests to create a partitioned table with a sliding window on an existing table.
So I created my partition function and partition scheme, and did a drop index with move to my partition scheme on the existing table. Everithing (selects, inserts,...) worked well, until I wanted to Alter my Partition function to do an extra RANGE SPLIT. Than I did get a Fatal Error 8646... error message. Fortunately, I remembered some problems with indexes and replication from the past who gave the same "8646" error message. And when looking at the table, I saw there was also an existing non-clustered index. After I dropped the non-clustered index, the problem was solved.
I tried to reproduce the same error with a smaller table, but didn't get any "succes".