I am "new" to K2, and just decided to give it a go. Im an experienced Joomla-administrator, but I am totally lost to this problem. Some info: I have a new installation og Joomla 2.5, and a fresh installation of K2. Right out of the box.

The problem:
When creating a new category, OR trying to import Joomla content, I keep getting this fatal error message:

Fatal error: Call to undefined function mb_internal_encoding() in/home/nettpakk/public_html/new/administrator/components/com_k2/tables/k2category.php on line 103

What is causing this?

Check the information tab on the k2 dashboard. It will probably report that mb_string is not enabled on your server. Your host can enable it or php.net/manual/en/book.mbstring.php if you have enough access you can do it

Thanks for info!

This is what the information screen is showing:
Web Server Apache/2.2.17 (Unix) mod_ssl/2.2.17 OpenSSL/0.9.8e-fips-rhel5 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635
PHP version 5.3.6
MySQL version 5.0.96-community-log
GD image library bundled (2.0.34 compatible)
Multibyte string support Disabled
Upload limit 8M
Memory limit 32M
Open remote files (allow url fopen) Yes

I host my own server with WHM/cPanel, so not sure how to enable it. I will try the link you provided, but hope I dont screw something up :)

Did not understand the installation-process, is there any command I can use (via SSH) to enable it?

Did you solve the problem? i have the same issue :S

Yes, actually I did. But I had to use EasyApache on my server to enable it. Actually I needed to rewrite Apache and PHP settings, and since it was my first time doing that - it took some time looking through all the options when rewriting.

By the link that William White provided above here, I also used this link for my server (wich has WHM/cPanel) to see what to do -> forums.cpanel.net/f5/enable-mbstring-whm-166670.html

If you are using WHM also, just go into your root login, type EasyApache in the searchfield, and start building a new profile (just use the default option and click "Start customizing...."). Just go through the step by step, and keep everything the way it is, except when finding the mbstring -button in there somewhere :)

Good luck!