I checked your site, but for now I’m not sure, why this error is appears. Seems like there is no plugin conflict, but probably there is some issue with sever configuration.
Can you give me FTP access, I will try check debug log on your site.
I enabled debug mode on your site and make some tests. But seems like there is no errors in debug log.
One thing, that I noticed – if i submit empty form (without files) and then add files (without reloading page) and submit again – seems like it works. Can you tell me, do you receive some files from test form last time?