In addition to having some downloads pausing and hanging when users attempt to download them, the new Mac App Store is also giving users an "unknown error 100" when they attempt to navigate.
This problem is likely another "break in" issue, and so far has been addressed by users in the following ways:
Restart the Mac
Some people have found that simply restarting their computer makes this problem go away. You might try rebooting into Safe Mode and then restart normally again to have the system run a few diagnostics and cleanup routines that may also help.
Sign out and sign back in
Other users have found that a more basic approach is to just sign out of the Mac App Store and then quit the program. After doing this, open the program and sign back in again.
It is possible this problem could be related to the handling of cookies. Open Safari and go to the "Security" section of its preferences. Then click the "Show Cookies" button and remove all cookies pertaining to "iTunes" or "Apple" (you can search for these), and then try the store again.
If you have any App Store tips or fixes, please let us know by e-mailing us or posting them in the comments. We look forward to hearing about your experiences.