Regular Font is set to bold

I want to use “TheSans” font from the “Thesis” font package ( You searched for Lucas-de-Groot • )

It is correlty installed and can be used by other programs as any other font. Exept emClient (pro):

I can select the font (Settings → Mail → Compose → Preferred font for messages ) but font (TheSerif-5-regular.otf), chosen as “regular” is always set to “bold”.

I only want to use it for typing in non html mode. But not as bold font.

What can I do to solve this?