Working during sick leave

I once worked from home during my sick leave. I was sick with flu, but we were in the middle of important project. I did not get meal vouchers for the days I worked!

If your doctor has given you a certificate that y ou are unfit to work, then you cannot go to work. If you go to work while under such a certificate you are not covered by insurance. If you wish to go to work, then you should ask your doctor to cancel the certificate.
Should you wish to do part time / some work while on sick leave, same applies (your doctor needs to approve this).
The HR department of wherever you work should confirm this.

Are you asking about working for your employer or doing work for yourself at home?
If you are unfit for work then you should not work for your employer and they should not allow you to work for them.
However, a sick note should specify whether or not you are restricted to the home - apart from visits to the doctor or hospital - or whether you are free to leave your house.
Whoever is paying your benefits has the right to check on you and you risk losing those benefits if you break the rules.