Howdy mydigitera,
That is a good question. Let me explain to you some of the inner workings of PHP to inform the problem. When you upload a file via a web browser to a PHP website it is stored in a special tmp folder. PHP gives give the script, in this case the Events Calendar, the name of that temporary file and it should give the script permission to access the file. However, sometimes it does not give the appropriate permission, or the file is not successfully uploaded and so PHP can not give a filename. When that happens PHP gives you this error.
Sometimes this is the result of a misconfiguration of your php.ini. However, if you it only does this for one file it is likely something about the file itself. Is the file too big? Is the filename causing problems? Does the file contain a virus and get blocked by a virus scanner? All of these can result in that error you see. The first place ot check is your server logs. You might also try renaming the file. Finally, if nothing else you can resort to google for reasons why the php function file_exists() is returning false for an upload.
I hope that helps you resolve this. Cheers!
– Brook
