Abgesehen davon, dass der Mangel sichtbar war, fehlte die Gutschrift.

It is a fixed expression meaning something like:

• apart from the fact that
• besides the fact that
• leaving aside that

So Abgesehen davon, dass der Mangel sichtbar war, ... introduces an additional point: not only was the defect visible, but something else was also wrong.

A useful way to think of it is:

• abgesehen von = apart from
• davon = from that / of that
• dass clause = a full clause explaining what that refers to

So the structure is idiomatic, not something you should translate word-for-word too mechanically.

Because German often uses a pronominal adverb like davon, darauf, damit, etc., and then explains it with a clause.

Here:

• davon points forward to the following idea
• dass der Mangel sichtbar war spells out that idea

This kind of pattern is common in German. Compare:

• Ich freue mich darauf, dass du kommst.
• Er spricht davon, dass ...
• Abgesehen davon, dass ...

So davon is part of the fixed expression, and the dass clause gives the content.

Because dass der Mangel sichtbar war is a subordinate clause inserted into the sentence.

German punctuation rules require commas around subordinate clauses.

So the structure is:

• Abgesehen davon, dass der Mangel sichtbar war,
• main clause: fehlte die Gutschrift.

This is very normal in written German.

Because dass introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• main clause: Der Mangel war sichtbar.
• subordinate clause: ..., dass der Mangel sichtbar war.

This is one of the most important German word-order rules.

Mangel is a masculine noun: der Mangel.

In this clause, it is the subject of war, so it is in the nominative case:

• der Mangel = the defect / flaw / deficiency

You are seeing:

• der Mangel = nominative singular masculine

If it were object case, the article might change, but here it is simply the subject.

Mangel can mean:

• defect
• fault
• flaw
• deficiency
• sometimes shortage/lack, depending on context

In this sentence, because it is sichtbar and something that can apparently be seen, defect or flaw is probably the most natural interpretation.

It is a fairly formal word and is common in technical, legal, quality-control, or business contexts.

Because sichtbar is not being used as an adjective directly before a noun; it is being used as a predicate adjective after the verb sein.

Compare:

• der sichtbare Mangel = the visible defect
  
  • adjective before noun, so it takes an ending
• der Mangel war sichtbar = the defect was visible
  
  • predicate adjective, so no ending

This is the same idea as:

• Das Auto ist teuer.
• Die Blumen sind schön.

After sein, adjectives do not take adjective endings.

fehlen usually means:

• to be missing
• to be absent
• to be lacking

So die Gutschrift fehlte means:

• the credit was missing
• the credit note was missing
• the refund/credit entry was not there

This is different from English, because German often uses fehlen where English might use there was no ... or ... was missing.

Because fehlen is not used like English to miss.

In English:

• I miss the train.

But in German, fehlen usually works more like:

• The train is missing (to me).

So the missing thing is often the subject.

In this sentence:

• die Gutschrift = nominative subject
• fehlte agrees with it

That is why it is die Gutschrift fehlte, not an accusative object.

Yes. fehlen very often appears with a dative person affected by the absence.

For example:

• Mir fehlte die Gutschrift. = The credit was missing for me / I did not receive the credit.

But in your sentence, no affected person is stated. The sentence is more impersonal and factual:

• fehlte die Gutschrift = the credit was missing

That sounds natural in business or administrative language.

Gutschrift is a feminine noun: die Gutschrift.

Depending on context, it can mean:

• credit note
• credit entry
• credit on an account
• refund credit
• credit memo

It is a business/finance word. In everyday language, the exact English equivalent depends on the situation:

• banking
• invoices
• bookkeeping
• customer refunds

So you should choose the English word based on context, but grammatically it is just a normal feminine noun.

German often puts an adverbial phrase or subordinate clause first in order to set the scene or emphasize it.

Here the sentence starts with:

• Abgesehen davon, dass der Mangel sichtbar war,

and then the main clause follows:

• fehlte die Gutschrift.

This fronting gives the sentence a structure like:

• Apart from that fact, ...

It sounds natural and fairly formal.

Yes. You could say:

• Die Gutschrift fehlte, abgesehen davon, dass der Mangel sichtbar war.

That is grammatically possible, though the original version sounds smoother and more natural in formal written German.

Starting with Abgesehen davon, dass ... makes the sentence feel more organized and argumentative: it introduces one point before giving the next one.

Not exactly.

• obwohl = although / even though
• abgesehen davon, dass ... = apart from the fact that / besides the fact that

obwohl introduces a contrast:

• Obwohl der Mangel sichtbar war, fehlte die Gutschrift.

That sounds more like: despite one fact, another fact was also true.

But abgesehen davon, dass ... usually adds another consideration rather than creating a strong contrast. It is more like saying:

• Besides that...
• On top of that...

So the nuance is different.

It sounds more formal and somewhat administrative/business-like.

Reasons:

• Abgesehen davon, dass ... is a fairly polished connector
• Mangel is formal/technical
• Gutschrift belongs to finance/business language

A native speaker would understand it easily, but it does not sound especially casual. In a conversation, people might choose simpler wording depending on context.