We have worked through all the older data for which Contact Grouping didn't work (mentions that entered our tool between 11:10 PM CEST and 11:58 PM CEST) and reindexed it.
During this reindexing process we gave priority to accounts that had users online. For those accounts we finished reindexing about an hour ago.
We have received no more complaints, and also no longer see traces of corrupt data, so can consider this incident closed.
The issue was caused by taking into production some behind-the-scenes changes. These changes will allow us to deliver new messages to your Inboxes faster, and we see promising results already; but by doing so we broke the Contact Grouping feature for a short while. All should be working fine again now, though.
Thanks for bearing with us while we solved this issue. Please contact firstname.lastname@example.org
if you would still encounter problems.