If you get the following error while using Google’s Gmail Service (gmail.com):

Oops? the system encountered a problem (#104)

..then you should try deleting your browser’s cache and everything will be resolved.

This appears to be a local problem and not on the Google servers as data is pushed via cookies to the local machine using the web service.

(See my deeper write-up on algorithms.)

In order to delete your browser’s cache in Firefox click Tools->Clear Recent History then select “Everything” in the dropdown menu and then deselect everything except for “Cache” in the below list box.
Click “Clear Now” to complete this task.