Hmm, tough one, since if the ip settings are messed, then everything can happen Those homeplugs also do not like power dividers and some bad ones do not work in seperate power groups.
I think this is a Double-DHCP problem. Your linksys is a dhcp server and your Fritz!Box is as well.
First off all i would recommend bringing all the homeplugs back to its factory default settings. Make sure your Fritz!Box accepts new DHCP connections from all kind of devices. (= default setting). Remove the computer names in the Fritz!Box dhcp that you do not recognize and/or are not working.
In the below scenario i do not use the LinkSys at all. I am keen to know if the Fritz!Box can see the homeplugs.
Do not connect any utp cables, just plug one of them in a 220v socket and push the reset button for 7 seconds.
Then plug in the other in a 220v socket and push the reset button for 7 seconds.
Now unplug them both. Hopefully this wil bring them back to factory default.
I am going to assume the Fritz!Box (a 7030?) has 4 UTP sockets, one of them is connected to your pc and 3 are free.
Connect a working UTP cable (NOT a cross-over!) from your Fritz!Box (awesome things by the way, i love my new Wireless N +Dect Fritz!) to one of those homeplugs.
Now first connect that homeplug with the utp to a 220V very very close to your FritzBOX, but NOT in a power divider. Use a wall 220V socket. Maybe it starts blinking that it found an IP adress. Hopefully you can see it in your computer/Fritz!Box settings page now.
Now connect the other homeplug without utp to a 220V very very close to your television, but NOT in a power divider. Use a wall 220V socket.
Connect a working UTP cable (NOT a cross-over!) from that power plug to your television.