Internal Server Error 500 on creating Quotes

I have installed InvoicePlane on my DigitalOcean droplet. It runs on Apache. Creating invoices and everything else works perfect but creating Quotes causes internal server error 500.
This is the exact error:
[25/Jan/2016:21:25:01 -0500] “POST /quotes/ajax/create HTTP/1.1” 500 1116 “” “Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.111 Safari/537.36”

Please take a look at the server error logs. This is not an error log, its the access log and not very helpful.