Thanks to everyone!
The solution in HTML purifier filters "target" atribute works for me.
I understand that it is an security issue, but the OJS behaviour is inconsistent. The new window links work in e.g. the footer, but not in the announcements (in OJS 3.1.2, OJS 3.3.0). In both cases, the TinyMCE is used.
Regards,
Carola