I know that First/First and First/Follow conflicts exist in a grammar which makes the grammar "not LL(1)". I was just wondering if Follow/Follow conflict exist in a grammar.
Yes, this is possible, but it requires an unusual configuration to make it happen. Consider the following grammar, which has been augmented with a new start symbol:
S' → S$
S → tT
T → A | B
A → ε
B → ε
Now, let's imagine trying to fill in our LL(1) parse table, which is shown here:
$ t
+----------+----------+
S' | | S' -> S$ |
+----------+----------+
S | | S -> tT |
+----------+----------+
T | T -> A | |
| T -> B | |
+----------+----------+
A | A -> e | |
+----------+----------+
B | B -> e | |
+----------+----------+
Notice that there are two items in the entry for (T, $). And that makes sense: if we have the active nonterminal T and see a $, we know that we need to select a production that's going to expand out to the empty string. And we have two different ways of doing this: we could use T → A or T → B, with the ultimate goal of expanding each of those nonterminals out to the empty string. This is a problem - we can't predict which route to take.
Now, what sort of conflict is this? It can't be a FIRST/FIRST conflict, because FIRST(A) = {ε} and FIRST(B) = {ε}, so neither A nor B has any terminals in its first set. It can't be a FIRST/FOLLOW conflict for the same reason.
That means that it's the rare FOLLOW/FOLLOW conflict - we know that we'd choose the production based on what's in the FOLLOW sets of A and B, and yet they're exactly identical to one another and so the parser can't choose what to do next unambiguously.