According to this and other things I've read, it could possibly be something on the server that was turned off accidentally.
I know that FX.php uses XML, so if by some chance XML deployment is turned off, this error would be returned.
I've never personally encountered this error, and do not use fx.php myself. But troubleshooting steps might include trying to find out if XML is running. stopping/restarting WPE. or even reinstalling filemaker server advanced.
I've had this error twice in the past several months on filemaker server 12 - Most recently today, even with the 12v04 patch.
Restarting the server did not work to fix it, restarting web services did not fix it.
I tried repairing the server installation using the latest patch and that did not fix it.
So either increasing the log space from 40mb to 60mb for web log fixed it (probably not... thought I tested the sites to see) or disabling xml web publishing, saving (and restarting web part) and turning xml pubishing on (and restarting web part) fixed it finally.
The logs themselves had nothing to say. The last logs were from a backup operationt that filemaker runs late at night, and tells me about the consistency checks being ok.
The first time we encountered this error we uninstalled filemaker, rebooted, and installed filemaker again, which was pretty terrible.
I've never had this problem on filemaker 10 server we're *still* running somewhere else.