With help of Lucas (i think) and a bit probing I found out that this bug occurs when one does not choose a [Topic], a category, like "Compile" for your bug report to live at.
I am not sure if anyone else is able to produce this bug - i think it is a bug in Mantis - but now that I know how to avoid it, I am happy.
We had some problems moving from Mantis 1.0 to Mantis 1.1 and Mantis 1.1.2 (latest stable) had some bugs that we couldn't live with. Some accounts may have got some invalid setting but resetting the settings seems to help. If anyone needs help with that post here or contact me.