Na sastanku se ne radi o velikom problemu, nego o malom kašnjenju u dostavi.

Because Croatian usually uses na with events, gatherings, and activities:

• na sastanku = at the meeting
• na koncertu = at the concert
• na predavanju = at the lecture

By contrast, u is more often used for being physically inside a space:

• u sobi = in the room
• u uredu = in the office

So na sastanku is the normal idiomatic way to say at a meeting.

Radi se o is a very common Croatian expression meaning:

• it is about
• it concerns
• it is a matter of

So:

• Radi se o problemu. = It is about a problem.
• Ne radi se o tome. = It is not about that.

Even though the verb raditi often means to work, in this expression radi se o has a different idiomatic meaning. You should learn radi se o + locative as a fixed phrase.

Here se is not really reflexive in the English sense of himself / herself / itself. It is part of an impersonal construction.

In radi se o..., Croatian uses se to make the statement less personal and more general, something like:

• it is about...
• the issue is...
• we are dealing with...

So in this sentence, se is simply part of the standard expression radi se o.

Both are possible, but word order in Croatian is shaped by clitic placement.

In your sentence:

• Na sastanku se ne radi o...

the clitic se comes early, after the first phrase Na sastanku, which is very natural Croatian word order.

You will also very often hear:

• Ne radi se o velikom problemu...

That is also completely natural.

What is less natural here is putting se later after ne radi if the sentence already begins with Na sastanku. So:

• Na sastanku se ne radi o... = natural
• Ne radi se o... = natural
• Na sastanku ne radi se o... = possible, but less typical

Also note that ne goes directly before the verb:

• ne radi

Because the preposition o requires the locative case.

So after o, the noun changes form:

• problem → o problemu
• kašnjenje → o kašnjenju

And the adjective must agree with the noun in gender, number, and case:

• velik problem → o velikom problemu
• malo kašnjenje → o malom kašnjenju

So the pattern is:

• o + locative

This is one of the key things to remember in this sentence.

Because Croatian uses nego after a negation when correcting or replacing one idea with another.

Here the pattern is:

• ne X, nego Y = not X, but rather Y

So:

• ne radi se o velikom problemu, nego o malom kašnjenju
• it is not about a big problem, but rather about a small delay

By contrast, ali is a more general but:

• Htio sam doći, ali nisam mogao. = I wanted to come, but I couldn’t.

So in this sentence, nego is the correct choice because the second part replaces the first after ne.

Here u dostavi is in the locative case.

That is because u takes:

• accusative for motion toward something
• locative for location or being within something

Compare:

• Idem u grad. = I’m going to the city. (motion, accusative)
• U gradu sam. = I’m in the city. (location, locative)

In kašnjenje u dostavi, the meaning is not movement into delivery, but a delay in the delivery process, so locative is used:

• u dostavi = in delivery / in the delivery process

It means a delay in delivery or a delivery delay.

More literally:

• kašnjenje = delay
• u dostavi = in delivery / in the delivery process

So this phrase refers to the process of goods being delivered, shipped, or brought to someone.

Depending on context, it could mean:

• a shipping delay
• a delivery delay
• a delay in supply/distribution

Kašnjenje is a neuter noun. Many Croatian neuter nouns end in -nje or -je.

Its basic form is:

• kašnjenje = delay

After o, it becomes locative:

• o kašnjenju

So in the sentence:

• o malom kašnjenju

you have:

• malom = locative singular adjective
• kašnjenju = locative singular neuter noun

This may look unusual to an English speaker, but it is a very regular Croatian pattern.

Because nego introduces a contrasting correction:

• not this, but that

Croatian normally separates that contrast with a comma:

• ne X, nego Y

So:

• ... ne radi o velikom problemu, nego o malom kašnjenju.

The comma helps mark the switch from the rejected idea to the correct one.

Yes, very common. It is a standard, neutral, everyday expression.

You will hear it in:

• conversation
• news
• business contexts
• formal explanations

Examples:

• Radi se o nesporazumu. = It’s a misunderstanding.
• Ne radi se o novcu. = It’s not about money.
• Radi se o hitnom slučaju. = It’s an urgent case.

So this sentence sounds very natural Croatian.

Yes. Croatian has several natural alternatives with a similar meaning. For example:

• Na sastanku nije riječ o velikom problemu, nego o malom kašnjenju u dostavi.
• Na sastanku se ne govori o velikom problemu, nego o malom kašnjenju u dostavi.

But radi se o is one of the most standard and useful ways to express this idea.

So for a learner, radi se o + locative is definitely worth mastering.