I shared a bit of trial and error and resolved a problem I was having with a Gravity Form here: http://www.gravityhelp.com/forums/topic/form-not-submitting-email-and-no-confirmation
As that thread is now closed (it was resolved) I thought I'd add something here so that one day a Google search might lead someone to a further glitch I've pinpointed.
I use WPHive which allows me to run multiple Wordpress sites off a single installation, single directory and single database. It was causing 404 header status errors, even though the page itself worked fine. It also meant Gravity Forms failed (NOT the fault of the Gravity Form plugin!).
Just when I thought I'd fixed the problem, it all failed again. I can have the WPHive plugin active, but if I add a new site, the 404 error and gravity form submit error returns.
So my workaround fix is like this:
1. Create a new site with WPHive, install it, log on.
2. Deactivate and DELETE the WPHive plugin (no sites vanish, the sites are stored in the database).
3. Refresh the permalinks by simply clicking SAVE without changing the setting.
4. If you want to add more sites, simply reinstall and activate WPHive and go back to step 1.
I know this is not really a Gravity plugin problem, but hopefully this little trick continues to work and will perhaps help someone else in the future.