Hello,
An advanced joomla user, with advanced joomla group level, upon editing a K2 item at front end, can choose only guest public and super special as content access level, whilst the registered and other custom groups are not available. Any ideas why?
Thanks a lot.

Hello,

Which versions of Joomla! and K2 are you using?

Sorry, always forgetting about that.
Joomla! 2.5.24
K2 v2.6.8

Your versions are a bit outdated. Update to Joomla! 2.5.28 and apply the security updates as well.

K2 should also be updated to 2.6.9

Let me know if that solved the issue.

Not sure if possible to update Joomla and K2 at this time, i need to search a lot to remember where I've done some important code modifications without proper overrides, any chance to solve that issue without updating entire site?
An by the way, when super administrator is editing K2 items at front-end all level access groups are available.
Thanks a lot in advance.

I'm sorry for bumping, but is there any chance to solve that issue without updating entire site?

Without any hassle no.
If this is not related to any 3rd party extension you need to manually patch K2. Locate the solved issues - github.com/getk2/k2/issues - and manually patch the code from the commit.