The solution to this problem was discovered in the course of solving my other problem, with Pulseaudio and Flash.
First I needed to completely purge and reinstall Pulseaudio. One way to do this is to run the command:
sudo apt-get --purge --reinstall install pulseaudio
Then, after I had a clean start, I discovered there was this odd entry in
/etc/pulse/client.conf. It had a line that said:
autospawn = now
It seems that
now is not a valid option. I have no idea where that option was coming from, as I thought I was starting fresh after reinstalling.
I changed it to
yes, and then commented it out as well anyway.
; autospawn = yes
Rebooted, and then I was able open
pavucontrol without errors.
The lesson I took from this is that purging an application might not be as complete as one would assume it to be.