A corrupt column was the culprit for me. This is what I did wrong:
- Created a single-line text column and added it to my document set
- Set it as a shared column in document set settings.
- Indexed the column.
- Ran script to update that column for all documents and document sets. (I think this is where the column broke because I forgot to add it to the document content type)
- No longer able to upload documents or edit properties in this library.
- "Fixed" the issue by deleting the column
EDIT: The error happened again! However, now I think the issue is being caused by indexing columns. This is what I did wrong this time:
- Created 3 new columns
- Ran a script to update those columns
- Created an index for the new columns while the script was running
- Created an index for an EXISTING single-line text column (NOTE: the script was running, but I was not updating this column)
- "Fixed" the issue by removing the index for the existing single-line text column
I don't think my script is the has anything to do with it considering this is the second time adding an index for a single-line text column has caused issues.
Will test and update my answer with results.