Export intenal error

Here we have configured odk aggregator on linux system and mysql server. everything working fine but when we are going to export the form and click on download on it then i am getting below error. please help me out for the same. thanks

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator at admin@wildseveodk.com to inform them of the time this error occurred, and the actions you performed just before this error.

More information about this error may be available in the server error log.

Apache/2.4.18 (Ubuntu) Server at server.wildseveodk.com Port 443

What version of Aggregate are you using? What do your server error logs say?

aggregate version is "ODK-Aggregate-v1.6.1-Linux-x64.run", i am not getting any log in server