You can get the code from the repo (DEV version) - github.com/getk2/k2 to ensure that you have the correct amount of == :)

Hi there, I have the same issue and I also confirm Miroslav solution works (thank you!!) but only for items.

I encountered this problem with PHP 5.6.19.

Changing to PHP 7.0.4 resolved the issue.

Issue is also in:
php : 5.6.21
jm : 3.5.1
k2 : 2.7.0

The solostation in my case was changing "===" into "==" on line 276 and 279

You can also install the DEV version - github.com/getk2/k2

Same at category.php

Did you install the DEV version?

I also had the same issue (no image is uploaded on item save) and solved by changing "===" into "==" on line 276 and 279 in the administrator\components\com_k2\models\item.php

My configuration:
php 7.0.6
joomla 3.6.0
K2 2.7.0

Same error but did not get solved updating to K2 v2.7.1 (which seems to have the proposed solution implemented). I have purged both joomla and K2 caches and checked tha all the relevant directories are writable and have the adequate permissions.

Config: Joomla 3.6.2 / PhP 5.6.22 / K2 2.7.1

Unfortunately I cannot move to PHP 7.0.x in my current hosting. I would really appreciate any suggestion...

Are you getting any errors?
Which other extensions are you using?