This is a generic (though fairly uncommon) error describing a connection issue between your browser and their database (I'm not an ffl, but I know a thing or two about web programming).
As more than one person reported this and you didn't say this form was significantly different than others you have submitted, it was most likely on their end. The server being busy could be the reason, but a request timeout error is more common for that. It could also have been that they changed a parameter on that page incorrectly and caused data to be submitted incorrectly. We had a situation at work the other day where certain pages would not work for people logged in with usernames that had a space in them. We made a change and forgot to properly parse usernames, the spaces then broke the code. They could have also made a change that altered how the page excepts data (there are two methods, GET & POST, and they can't always be used interchangeably).
There are only four things you can really look at on your end if that particular error happens. Clear your cache and cookies, make sure there are no fields that exceed the maximum length allowed, no files attached larger than allowed (don't even know if that is possible in this instance), and completely close all browser windows then restart the browser (not the computer, just the program). The last step would close any sessions you had running ang create new connections to any site you were using. The cookies and uploaded files are the most likely culprit if the error is from the client side (your browser). The field length issue is possible, but a stretch (normally this would be a different error, but could be depending on how they coded the page). If all of those are fine on your end, then the error is their system and is not really specific enough to tell you what it is. Also there would be nothing you could do but wait to see if it resolves or contact them to let them know. In this case it seems to have resolved (at least for now).
Web errors (404, 500, 413, etc...) are generally about as useful as you telling you doctor you have a cough. This one has a little more info that would allow somebody with direct access to the server to get a little more info, but it would also be cryptic and mostly useless.