Hi
I have PHP notice and warning:

PHP Notice: Constant K2_JVERSION already defined in /public_html/dfk/plugins/system/k2/k2.php on line 23
PHP Warning: Cannot modify header information - headers already sent by (output started at /public_html/dfk/plugins/system/k2/k2.php:23) in /home/joachimm/public_html/dfk/components/com_community/libraries/core.php on line 577

I do not know what's going on.
This does not work well for adding a new articl like this. i.imgur.com/foJt9P1.jpg
Sorry for my english ;)

K2 2.8
Joomla 3.8.1
JomSocial 4.4.4

Regards, Joachim

Try updating or disabling your JomSocial plugins and modules one by one to see if you can find the culprit.
There are posts in their forum about this.
If things are working properly you can set error reporting to none