Text balloon over plugins in admin and k2-sbm-loader.php

Sorry, I have tried for ages to find this, and I’m sure it’s covered somewhere, but I can’t find it.

I just upgraded to 2.6 and used the upgrade assistant plugin. It worked fine, but I did get an error message about a k2 component although it all looks normal. But now I have the number 1 in a small text bubble over the Plugins tab in Admin. I can’t see any of the plugins have problems, as they are all activated. What does that bubble mean, and is there a known issue with K2 upgrades (the error was this: The Plugin ../themes/k2/app/includes/k2-sbm-loader.php could not be activated succesfully. You will need to activate it manually.)

But I can’t see anything like that in my plugins.

Can anyone help?