Error. To see why that happened, laravel have log file. It is located in folder

Code:
app/storeage/logs/laravel.log
First delete that file. Then generate the error. Then look at that file, read it. You will find why that error happened. Try to find the reason by reading error log.

EDIT: We delete the file, so we have less logs to read. If you don't delete, the file may have lot more lines. So its better delete the file, generate the error by uploading the image or what ever step required to reproduce the error, then read the log file.