This has just recently been reported by others.
For More Information see: XML error: Invalid character at line 1
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
I saw the other issue, and linked to it in my notes. But it's not quite the same issue, and this time the issue goes away on rebooting the machine. It's another valuable data point for diagnosis as you can find out what the difference between the two states ( before and after reboot ) of the single machine is, and thus more easily locate the problem.
Thank you for your posts.
Most definitely send in the FileMaker Pro and PHP files. Anything that helps us replicate the issue is appreciated. Check your Inbox at the top of this page for instructions where to send the files.
Thank you for your files.
Our Testing department has been able to reproduce the issue under Windows Server 2008, but not under Mac OS X 10.9. Your files, along with the Testing department findings, have been sent to Development for review.