Learn Dutch: Het ziekteverzuim stijgt; de werkgever is dan ook bezorgd. - Sick leave is rising; the employer is therefore worried.

Both can translate to "therefore" or "so", but dan ook adds a specific nuance. It implies that the consequence is entirely logical, natural, or to be expected. Because sick leave is rising, it makes perfect sense that the employer is worried.

Could I place dan ook at the beginning of the second clause, like we do with "therefore" in English?

No, the particle cluster dan ook is typically placed in the "middle field" of the Dutch sentence, meaning it comes right after the finite (conjugated) verb and any pronoun subjects. Here, it sits directly after the verb is. If you wanted to start the clause with a consequence word, you would use dus (...dus is de werkgever bezorgd).

What exactly does the word verzuim mean in the compound noun ziekteverzuim?

Verzuim means "absence", "default", or "failure to appear" (from the verb verzuimen, meaning to neglect or fail to do). Combined with ziekte (illness), ziekteverzuim specifically refers to "sick leave" or "workplace absenteeism due to illness".

Why isn't the word order inverted in the second part of the sentence after the semicolon?