I updated K2 to version 2.8.0. and just after saving the item remains a white page and the item is not saved (joomla 3.8.1).
So I went back to 2.7.1 and it's okay. Version 2.7.1. but he has the mistake of not being able to find Czech characters in the administration.

Please see here www.joomlaworks.net/forum/k2-en/48113-k2-2-8-is-it-a-security-release,-really#163466
You should upgrade to 2.8
There is probably a plugin interfering with the save/update.
There are also several threads here about the same kind of problem

Enable error reporting to see the PHP actual error.

The problem was caused by the Png images for K2 items plugin, which loaded the class.upload.php file from the K2 component. But the reason was not in the plugin, but in the fact that K2 developers changed the file structure and moved this file (along with others) to the media / k2 / .... directory. Thus, when applying the plugin at the same time, there should be two classes with the same upload name, which PHP did not like and returned the 500 error. So it was enough to change the file path to the file and it all works again.
However, in my opinion, the installation / update process K2 would have to delete these shifted / unnecessary files!

png4K2 has been updated to play nicely with K2 2.8.x. You need to update that extension in your installation.

But the update to this plugin is not available in Joomla.
Even the JED is no longer available - extensions.joomla.org/extension/png-for-k2-items/

www.joomlaworks.net/forum/k2-en/46953-png-for-k2#164545

thanks

You 're welcome Miroslav :)