This error can happen in a number of different places across all plugins. In short, the plugin has tried to open a file on your disk and not been able to.

The actual cause should always be displayed as part of the error dialog itself and will usually help you to work out what is wrong. Common causes would be that the file simply doesn't exist if you're trying to read it, is already in use by another program, or if you're trying to write then the place it's being written to is not allowable or your disk is full. Using a different filename if you have a choice, or clearing some space on your disk if it is full, will fix this problem.