I suggest posting this in Report an Issue as a possible bug to see what the TS people have to say about it. (See tabs at top of this screen.) I can recall at least one similar report there, from memory but not how it was resolved.
I have exactly the same configuration here but a few machines are still XP. System is used very heavily 6 days a week and have yet to see this issue. We do have to re-install once in a while, especially on the XP machines as FileMaker 10 seems to "Mess up" the OS's ability to manage multiple open windows and have learned to re-install when we starting seeing issues that keep multiple windows from opening, but haven't seen issues with this particular error.
I haven't had this exact problem before, but I had other unexplained problems from older software and in my case it had to do with my anti-virus quarantine a file it shouldn't (No virus). You might try excluding the dictionary from virus scans.