telephone instead of brackets

I have a problem adding a mathematic formula to my text. When I open the formula-Editor, I see telephones at places where I expect to be brackets. You can see it in this picture:

How can I fix this?

I would be very thankful for any kind of help!

(edit: activated screenshot)

Which LO version you use? looks very similar to tdf#76239, which is fixed in 4.4.7+