Hello and
Christof
Very hard to tell, but can you scrape them off with your fingernail? Have you got a magnifying glass? If you scrape some off and look at them through a magnifier, you should be able to see if they are insects or just natural growths. (you might be able to get a picture through the magnifier so we could see better, too)
The way they are situated, the only thing I could think of was scale insects, but they don't usually go out onto the upper sides of the leaves like that. Stems and undersides of the leaves are where they like to attach and suck the juices out of your plant. Are there any underneath the leaves?
If that's what they are, a horticultural oil spray is the remedy. Scale insects protect themselves with a little shell so direct contact sprays like soapy water don't work, except on the nymphs. The oil smothers them.
