Thanks, hoppie, everything works now! I found some code in bbPress that looks like it’s meant to warn users when the PHP MySQL libraries are missing, but I wasn’t getting the warning message. I’ll look into filing a bug report about this.
Hmm, it doesn’t look like I have the PHP MySQL libraries installed. I would expect that to generate a meaningful error message, but perhaps not. I’ve asked my administrator to install the libraries, and I’ll let you know if it helps.