To really know, you'd have to debug the software. However,
fopen is a standard C libary function (see e.g. http://www.gnu.org/s/libc/manual/html_node/Opening-Streams.html#index-fopen-929 ), which is used to open a file for reading or writing.
The error message you see is often printed if a program invokes
fopen, and the invocation fails. Properly behaved programs would also print out which file they tried to open, and what kind of failure occurred, but this program is apparently not one of those :-).
So most probably it means that the program tried opening a file, but could not (file not found / rights problem / disk on fire / whatever).
To investigate further, you could run the program using
strace ( https://wiki.ubuntu.com/Strace ) , this will (more or less) log all system calls a running program makes. There you can look for a call to fopen right before the error message, that should be the culprit. strace's log output will show the filename.