I just installed K2. First time. K2_v2.1_build20090909.zip
I run into the same problem when creating a new category. When I click on save, it jumps to a http 400 error.
Turning on the debugging in Joomla does not help, cause it jumps straight to the errorpage...
I cannot find any other build then the last one here on K2, so I cannot test a previous build.

If this problem is solved, or known. Can anybody please post a solution.

Ty

SOLUTION:
Sorry, for asking the question first and then come up with the solution. (I was reading and playing around with this issue for hours... Then found the solution within minutes after posting)

Not my solution by the way, read it somewhere in the forum...

To solve this problem: install the extension: mbstring in your php. It is not a default extension.

Steven v C said:I just installed K2. First time. K2_v2.1_build20090909.zipI run into the same problem when creating a new category. When I click on save, it jumps to a http 400 error.Turning on the debugging in Joomla does not help, cause it jumps straight to the errorpage...I cannot find any other build then the last one here on K2, so I cannot test a previous build.If this problem is solved, or known. Can anybody please post a solution.Ty

I'm getting this error: Fatal error: Call to undefined function mb_regex_encoding() in /home/mediasph/public_html/ipl/administrator/components/com_k2/tables/k2category.php on line 50

Does installing it on a subdomain cause problems?

The solution provided here (installing the library mbstring) works perfectly. I am using Apache on my local PC. I also checked and the server where I am going to install the live version has the library installed too, so that's great.

Thanks for the info!