When I switch sequence between FK and IK everything works fine but if i reload Houdini then the bone structure breaks down https://monosnap.com/file/NV26XAj02QY5BJGBcWtjbagNcvzjmc [monosnap.com]
Why is this happening? Am I doing wrong?

can you provide your hip file and hda?

goldfarb
can you provide your hip file and hda?

Of course, thank you! I'm attaching these files here. In order to see that the skeleton breaks you need to move this slider to zero https://monosnap.com/file/Icyz0EmFRow2mE4TJ29LMV82PpReb7 [monosnap.com] save the scene and reload the Houdini.

first I'd recommend updating to Houdini 17.0.459 the latest production build or Houdini 17.0.478 the latest daily build, as there have been some updates to constraints.
second, I'm not sure what you're trying to do but that isn't how Blend Objects are used, in your example you have only one input to the blend object, so it will blend between the origin and the input
I think you want to have 2 inputs - I use nulls set at the locations of the two objects I want to blend between, then the output feeds into the object I want to control.
see the example file attached