Discussion boards -> Set an author of anonymous posts -> Top of the page says “Failed to process de-anonymization selection”
This is a known issue for Blackboard 9.1 SP13. While there is no patch of fix yet there is the following workaround.
Set all threads to "No, leave messages as anonymous" from the pop-up prompt. Then in List View of the forum, change each thread author using the "Change Author" option from the "Thread Actions" menu.