Can my boss contact me about work during sick leave?

It is not forbidden for an employer to contact an employee who is on sick leave. In fact, the employer has an obligation to contact the person on sick leave after a certain period of time to check how the individual is doing and to draw up a rehabilitation plan for returning to work. This should be done in consultation with the individual, the social security office, the doctor in charge, etc.
However, the employer cannot expect to receive answers to job-related questions during your sick leave. Consequently, when you are on full-time sick leave, you are not obliged to work and do not have to perform any work for the duration of your sick leave, in accordance with the Sick Pay Act (1991:1047). You have the right to refer to the fact that you are ill and unable to respond.
If there is a safety representative at your workplace, you can take support from that person in your contact with your manager.