My hosting guy fixed this by making the following changes in php.ini..

safe_mode = off
upload_tmp_dir = "/home/logik134/public_html/tmp"
date.timezone = CET
session_save_path = "/home/logik134/public_html/tmp"

Months later. I keep coming back to this thread. Today I installed the recent version of K2. This problem is still there.
Is there no K2-person who would be able to explain the issue and give a serious hint about a solution?

I need to attach files to elements, and I cannot, because of this error unless I upload each file individually.

Luciano wrote: Hello, a big THANKS to all JW crew, you guys rocks! :lol:

Yes, same issue here. :huh:

"Invalid backend response. Data is not JSON."

Fresh Joomla 1.7.2 + K2 2.5.1 setup!

KingHost Shared plan specs:

Web Server Apache/2.2.15
PHP version 5.2.17
MySQL version 5.1.56
GD image library bundled (2.0.34 compatible)
Multibyte string support Enabled
Upload limit 100M
Memory limit 128M
Open remote files (allow url fopen) Yes

Browsers:
FireFox 7.0.1
IE 8.0.6001.18702

SOLVED!!!!! - Now running Joomla - 2.5.9 and K2 - 2.6.5

Hi,

After a lonnng try/error/try/error.. I solved this issue!

Was needed 2 steps.

FIRST:
The problem of server reporting. I need to turf off this feature at my Joomala backend config. Your configuration.php must have the right chmod to change it.

- Log @ backend > Go to System > Global configuration > Server - Set "Error reporting" to "none".
- SAVE IT.

Thanks to
getk2.org/community/New-to-K2-Ask-here-first/520-Cant-Browse-Server-invalid-backend-response?limit=20&start=40#166108


SECOND:
Go to file: "elFinderVolumeDriver.class.php"

File Directory: on local host or web hosting search:

/administrator/components/com_k2/lib/elfinder

File: "elFinderVolumeDriver.class.php"

Open the file with an editor:

Go to line 752 where the line "$this->_checkArchivers();" should be! And disable this line by adding double slashes "//" (Example: //$this->_checkArchivers();)

Thanks to
getk2.org/community/New-to-K2-Ask-here-first/159715-RESOLVEDInvalid-backend-responseData-is-empty#159730