Deviation plugin - cannot enter values

[prospero68]

Hello everyone,
it seems like an eternity that I have installed the deviation plugin, but only recently did I try to get it working. So I went round in circles on a quiet day, took down the values and sat down to begin working on a deviation table.
But the plugin simply doesn't want to accept my values and keeps telling me instead, "Error Values entered are not valid"
For example, when going due north according to GPS COG I entered 0.0 as a true bearing and the fluxgate sensor's HDM value of 353.0 as a compass bearing. The plugin correctly calculates a deviation of 7. On clicking OK a window pops up saying the values are not valid.

Where's the mistake? Or maybe even the whole project is impossible?

I.e. the goal of my work would be to make openCpn correct the HDM values provided by the fluxgate sensor directly without a compass to display the boat's true heading in comparison to COG.
Did someone have similar problems - ore even solved them?
Thanks for any hints,