Like always, wolfden guiding in the right direction.
I've found the solution, i'll post here so can be of use to anyone.
After upgraded sabayon, sun's java was updated too. Somewhere changed the JRE engine to JAVA-JDK and java JDK DOESN'T have a jre. That's why mercury im was giving me that error.
I solved it running java-config -L (from console, just like wolfden likes) to see if there are any JRE available and notice that sun's JDK was marked as current jre.
then runned java-config -S # where # was the number of the sun's JRE or blackdown's or any other jre installed.
That solved it.