I think there may be a configuration problem with the mysql database underlying the Hypnos store.
I need to do a full backup of all our data (items, customers, categories and so on), uninstall the shopping cart software, upgrade the mysql version, then reinstall the shopping cart software, apply all shopping cart version updates, import all the backed-up data, and then re-insert all the modifications (logos, front page "Hypnos Recommends" messages and so on).
This is a big job and I'm not going to start it this evening. In the mean time, the store seems to be "touchy" as far as displaying errors here and there, but it's still secure and orders are still coming through, so it's fine to use it if you want to order something.
Sorry for the inconvenience, everyone!