Update after more context was added:
Die Front der Lumpen aufrollen
is not a commonly used idiom in the German language.
You have to take it apart and analyse its components: see the old part of my answer for this.
Is this translation accurate?
a. Nur ein grober Fehler der Verleumder ermöglichte es mir, die Front der Lumpen aufzurollen.
b. Only a crass mistake on the part of my slanderers made it possible for me to unravel the solid line of these worthless bastards.
Judging from the context you added, I assume a political context (see also Janka’s comment).
The sentence is problematic because the wording in German is unusual / unidiomatic.
It seems to me that the image chosen by the writer of the letter does not quite fit what he wanted to express, but I can’t be sure.
That makes translating it difficult.
I assume, the author wants to express that he uncovered who was behind the conspiracy, but the German expression implies that he also took action in some way, restoring his reputation (or to a similar effect).
As to your translation:
There’s no solid in the German sentence (a front may be solid, but not if used figuratively), and worthless bastards seems too strong.
But you can make up your own opinion whether your phrasing fits the intended style.
Old answer:
Without further context, I assume that the situation described in the letter means something like this:
There are two (or more) groups of people.
The writer of the letter belongs to one of the groups.
His group plans to take some sort of action against the other group.
This may be violent or nonviolent.
For an actual translation, more context is needed.
Explanation:
The partial sentence
Front der Lumpen aufzurollen
lacks context.
It is thus hard to tell its meaning, but we can analyse the structure:
eine Front aufrollen
is actually a fixed expression: aufrollen is quite often used in combination with Front.
In the military sense (or in the sense of violent conflicts of groups), it means
von der Seite her angreifen und zum Weichen bringen (DWDS)
This is to say:
attack an enemy line from the side to cause retreat [or to make a break through].
It is also used figuratively for other situations, such as non-military or nonviolent conflicts.
This expression does not have the same meaning as the verb aufrollen alone, which just means to coil or unfurl sth. or, figuratively, to treat sth. again (discussion, case at court etc.).
Lumpen (sg. Lump) means (Grimm):
gewöhnlich in der bedeutung eines in abgerissener kleidung einhergehenden, daher armseligen, erbärmlichen menschen, zugleich von niedriger gesinnung;
Lumpen are rags, but it is also derogatorily used to denote people of lower class (because they don’t own anything but rags; see Lumpenproletariat on Wikipedia: the underclass devoid of class consciousness).
It is also very generically used for people the writer doesn’t like. »Du Lump!« could be translated as »You scoundrel!«
