Reading many japanese sites and trying to understand presented info I want to share with you some (warnings on rubbish compilation, mainly because english nor japanese aren't my native languages
Fellowship limit break quest starts after speaking to Diederik NPC in Selbina, along the coast. If he doesn't give you quest - you might want first to cap your NPC level to 50.99 and also complete quest with finding ??? for your NPC in Promyvions, as discussed earlier.
Then he presumably lets you choose on of the two:
1) Go to Angelica in Windurst Waters second floor of Rarab Tail cottage or what was that house called.
2) Go to Umberto in Bastok Markets.
For now someone tried the way going to Umberto first.
After talking to him, get pickaxes and go using it on mining points in Koroloka Tunnel untill you dig "the shell of the val shellfish". Hand it to Umberto.
After that visit Diederik in Selbina.
Now we go to Castle Oztroja east courtyard, through some door to ???, inspecting which spawns Black Yagudo to fight. After killing it we inspect ??? again.
Report to Umberto (and Angelica?).
Fellow NPC now should be able to reach lvl55, congratulations!
Sorry for incomplete and rubish info again - had to improvise a bit, need yet to try everything myself once I wake up tomorrow -curses maintenance-