I'm using K2 2.7.0 and Joomla 3.6 in a Server Apache widh PHP 5.6.19.
When I try to insert the category image, I can't open the "Browse Server" box to search the image and upload it. I only upload from PC.
In K2 Items I can upload via "Browse Server"!
From console of my browser I receved the next error:
Uncaught ReferenceError: SqueezeBoxSqueezeBox is not defined k2.js?v=2.7.0&sitepath=/rentest/:319
(anonymous function) @ k2.js?v=2.7.0&sitepath=/rentest/:319
dispatch @ jquery.min.js:3
r.handle @ jquery.min.js:3

How can I solve my problem?

Thank you for reporting this.
It has been resolved - github.com/getk2/k2/commit/dac7cfad960d1c33914aa1555aa1944a28afe592

Sorry, but the problem isn't solved...
Here is the console:
Uncaught Error: Syntax error, unrecognized expression: [href=#catItemOptions] jquery.min.js:2
fa.error @ jquery.min.js:2
fa.tokenize @ jquery.min.js:2
fa.compile @ jquery.min.js:2
fa.select @ jquery.min.js:2
fa @ jquery.min.js:2
fa.matches @ jquery.min.js:2
n.filter @ jquery.min.js:2
z @ jquery.min.js:2
filter @ jquery.min.js:2
(anonymous function) @ k2.js?v=2.7.0&sitepath=/rentest/:86
dispatch @ jquery.min.js:3
r.handle @ jquery.min.js:3

Uncaught ReferenceError: SqueezeBoxSqueezeBox is not defined k2.js?v=2.7.0&sitepath=/rentest/:319
(anonymous function) @ k2.js?v=2.7.0&sitepath=/rentest/:319
dispatch @ jquery.min.js:3
r.handle @ jquery.min.js:3

Did you apply the patch and clear your site's cache?
I applied it and the browse button is now working properly
imgur.com/a/JJ8Er

Yes, but doesn't work..... :(

Sorry, now it works! Thank you!! :)

Sorry, now it works! Thank you!! :)

Nice to hear this :)