I had this same problem yesterday on my stock A500.
I updated a number of apps and then found I couldn't access any of the app folders (multimedia, entertainment etc) anymore. Kept getting an error about Acer Content Folders (process app.folder or some such) requiring a force close.
Force close didn't help though and neither did several reboot.
Was about to uninstall all my apps but thought I take a look online first and found a post somewhere on xda which might help.
Try going into Settings > Applications >Manage Applications > All
Go to Acer Content Folders
Hit Force Stop
Hit Clear Data (didn't remove apps or anything, just the app icons from my folders as far as I can tell)
Hit Clear Cache if its highlighted
Exit
After all that, my content folders are back to normal and I haven't had any problems since, even without uninstalling any apps.
HTH