When I click [Components] - [K2] I end up in items instead of the K2 control panel.
Plus: the tab bar with [Items], [Categories] etc is gone.

I'm lost, how can I get this back? Any help is greatly appreciated.

William

Just a thought, try turning error reporting to highest and see if errors are generated, may give you a clue

Hi William,
Thanks for responding, I'm sorry, you mean in htaccess or some place else?
Can you give some more info please? Thanks a lot!

Joomla - Global Settings - Server Tab - Error reporting

Thanks again :-)
I get this:
Strict Standards: Declaration of JParameter::loadSetupFile() should be compatible with that of JRegistry::loadSetupFile() in /home/spktmp/domains/tmp.nl/public_html/libraries/joomla/html/parameter.php on line 512

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/k2/k2.php on line 320

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sh404sef/sh404sef.php on line 30

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/administrator/components/com_sh404sef/sh404sef.class.php on line 27

Strict Standards: Redefining already defined constructor for class sh_Net_URL in /home/spktmp/domains/tmp.nl/public_html/administrator/components/com_sh404sef/sh_Net_URL.class.php on line 124

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/administrator/components/com_sh404sef/sh404sef.class.php on line 290

Strict Standards: Creating default object from empty value in /home/spktmp/domains/tmp.nl/public_html/administrator/components/com_sh404sef/sh404sef.class.php on line 421

Strict Standards: Creating default object from empty value in /home/spktmp/domains/tmp.nl/public_html/administrator/components/com_sh404sef/sh404sef.class.php on line 421

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/sh404sefcore/sh404sefsocial/sh404sefsocial.php on line 25

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/administrator/components/com_sh404sef/helpers/extplugins.php on line 94

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/administrator/components/com_sh404sef/helpers/extplugins.php on line 56

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/k2/k2.php on line 27

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sh404sef/sh404sef.php on line 144

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/shmobile/shmobile.php on line 64

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/k2/k2.php on line 120

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/offlajnparams/offlajnparams.php on line 49

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sticky_toolbar/sticky_toolbar.php on line 24

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sticky_toolbar/sticky_toolbar.php on line 26

Strict Standards: Non-static method DojoLoader::r() should not be called statically, assuming $this from incompatible context in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sticky_toolbar/sticky_toolbar.php on line 28

Strict Standards: Non-static method DojoLoader::getInstance() should not be called statically, assuming $this from incompatible context in /home/spktmp/domains/tmp.nl/public_html/plugins/system/dojoloader/loader.php on line 53

Strict Standards: Non-static method DojoLoader::addScriptFile() should not be called statically, assuming $this from incompatible context in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sticky_toolbar/sticky_toolbar.php on line 30

Strict Standards: Non-static method DojoLoader::getInstance() should not be called statically, assuming $this from incompatible context in /home/spktmp/domains/tmp.nl/public_html/plugins/system/dojoloader/loader.php on line 73

Strict Standards: Non-static method DojoLoader::addScript() should not be called statically, assuming $this from incompatible context in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sticky_toolbar/sticky_toolbar.php on line 38

Strict Standards: Non-static method DojoLoader::getInstance() should not be called statically, assuming $this from incompatible context in /home/spktmp/domains/tmp.nl/public_html/plugins/system/dojoloader/loader.php on line 61

Strict Standards: Only variables should be assigned by reference in /home/spktmp/domains/tmp.nl/public_html/plugins/system/sh404sef/sh404sef.php on line 162

Strict Standards: Non-static method DojoLoader::getInstance() should not be called statically, assuming $this from incompatible context in /home/spktmp/domains/tmp.nl/public_html/plugins/system/dojoloader/dojoloader.php on line 27

There are no fatal errors.
Turn it back off

Have you installed something just prior to this happening or did it ever work properly
If it did - look for why
if it never worked properly
Back up your site
reinstall k2

I'll uninstall and re-install... let's see what happens ;-)

After some more research I found a working solution:
answers.webhostinggeeks.com/joomla-extension-manage-tab-missing-1555

You have disabled the "Admin sub-menu", and now you’re not able to reach the entire toolbar/menu.
Looked at a different installation of Joomla and navigate to the extension manage page. Then copy the end of the URL ‘administrator/index.php?option=com_installer&view=manage’ and paste it at the end of the regular domain. Then you can enable the Admin sub-menu again.

Yes, I know, i screwed up... Thanks a lot for your help :-)