Stop sending email - Internal Server Error

Hello
I'm a Opencart version 2.3.0.2 user from Poland. When attempting to send an e-mail to users, the shipment stops at 90 with the message Internal Server Error. The error log does not show any error. How to solve this problem? Screen shot attached.

See the most recent server access logs events from your host console or from your FTP's access logs folder.

I found only such entries in the server logs
95.48.201.98 - - [05/Jul/2018:11:19:41 +0200] "POST /admin/index.php?route=marketing/contact/send&token=0jHXiFLivlkPiIV1wBQ5FuQ0f9Xm77Oa&page=11 HTTP/1.1" 500 - ".../admin/index.php?route=marketing/contact&token=0jHXiFLivlkPiIV1wBQ5FuQ0f9Xm77Oa" "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:61.0) Gecko/20100101 Firefox/61.0" /www/.../www/.../admin/index.php 114057 pid=3837 id=Wz3irVZv8d0AAAbJ5j8AAACP
... 95.48.201.98 - - [05/Jul/2018:13:23:59 +0200] "POST /admin/index.php?route=marketing/contact/send&token=0jHXiFLivlkPiIV1wBQ5FuQ0f9Xm77Oa&page=11 HTTP/1.1" 500 - ".../admin/index.php?route=marketing/contact&token=0jHXiFLivlkPiIV1wBQ5FuQ0f9Xm77Oa" "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:61.0) Gecko/20100101 Firefox/61.0" /www/.../www/.../admin/index.php 161710 pid=25779 id=Wz3-z1Zv8d0AAAfUK1YAAAIW

Are you using SMTP or sendmail? If using SMTP, try with Sendmail and see if that rectifies the issue.

.../admin/index.php?route=marketing/contact

should likely more look like this:
admin/index.php?route=marketing/contact/
to not produce a 500 (not found) Error.
Ernie
/www/.../www/.../admin/index.php

I changed to sendmail and everything works.

There we go!