Otkaz radionice bio je problem.

The subject is otkaz.

You can tell because:

• otkaz is in the nominative singular
• the past form bio is masculine singular, and it agrees with otkaz
• radionice is not the subject; it is dependent on otkaz

So the basic structure is:

• otkaz radionice = the cancellation of the workshop
• bio je problem = was a problem

Because after the noun otkaz, Croatian uses the genitive to mean of something.

So:

• radionica = workshop, workshop session
• radionice = of the workshop

That means:

• otkaz radionice = cancellation of the workshop

This is a very common Croatian pattern:

• noun + genitive noun
• for example, kraj sastanka = end of the meeting

So radionice is not a direct object here. It is a genitive modifier.

Here otkaz means cancellation.

But this is a useful word to watch out for, because otkaz can mean different things depending on context, such as:

• cancellation
• dismissal/firing from a job
• notice/termination
• sometimes even failure/breakdown in technical contexts

In otkaz radionice, the context makes cancellation the natural meaning.

Because bio has to agree with otkaz, which is a masculine singular noun.

Croatian past forms agree in gender and number with the subject:

• masculine singular: bio
• feminine singular: bila
• neuter singular: bilo
• masculine plural: bili
• feminine/neuter plural: bile in standard Croatian for feminine plural, and often bila for neuter plural in older/literary patterns, though learners usually first focus on the singular forms

Since otkaz is masculine singular, the sentence uses bio.

Bio je is the past tense of biti = to be.

In this sentence:

• bio = been / was
• je = is, but here it functions as part of the past tense construction

Together they mean was.

So:

• bio je problem = was a problem

Croatian past tense is often built with:

• a past active participle, here bio
• plus a present-tense form of biti, here je

Both word orders can occur in Croatian, depending on style, rhythm, and emphasis.

Your sentence has:

• Otkaz radionice bio je problem.

A very common alternative is:

• Otkaz radionice je bio problem.

Both are understandable and natural. Very broadly:

• je bio often sounds more neutral/conversational
• bio je can sound a little more formal, literary, or stylistically shaped

Croatian word order is more flexible than English word order, so learners will often see both patterns.

Problem is in the nominative singular.

After biti = to be, Croatian commonly uses a predicate noun in the nominative:

• To je problem. = That is a problem.
• On je učitelj. = He is a teacher.

So in your sentence:

• otkaz = nominative subject
• problem = nominative predicate noun

This is one of the basic sentence patterns in Croatian.

Because Croatian does not have articles like English the and a/an.

So:

• otkaz can mean a cancellation or the cancellation
• problem can mean a problem or the problem

The exact meaning depends on context.

That is why a short Croatian sentence can look more compact than its English equivalent.

No. It is a natural and concise way, but not the only one.

You might also see:

• otkazivanje radionice

This also means the canceling/cancellation of the workshop, but it focuses a bit more on the act or process of canceling.

A rough difference is:

• otkaz radionice = the cancellation, as a fact/result
• otkazivanje radionice = the act/process of canceling

In many contexts, both work.

Yes. Croatian allows several word orders, often with slightly different emphasis.

For example:

• Otkaz radionice bio je problem.
• Otkaz radionice je bio problem.
• Problem je bio otkaz radionice.

These all express basically the same idea, but the emphasis changes:

• starting with otkaz radionice focuses first on the cancellation
• starting with problem highlights the idea of the problem itself

This kind of flexibility is very normal in Croatian.