The 404 thing, any chance you have a folder on your computer known as OEM Computer, or that is the name of the computer itself, or perhaps maybe even a username?
If you copy and paste things outside your web (or from another FP web, the program will not update some of the URL's, or rather link through a filename as opposed to http://
You have to keep your eyes on these sort of things with FP because the program will do some things automatically, without notifying you.
What version of FP are you using?
There is an option on both progs (diff. places) that will allow you to check your links. What I done before finding out this wee prob is copy and paste your web to a new location so any "filenameS" that are used as links are now broken. Then you can let the program automate the task of finding what ones should be changed.
If you have FTP'ed your site before, this has been known to affect the FP components, including forms. If the FP components were not working, then you'd see this message. Try making a new form on a new page maybe? See if the same thing happens.