It seems that the main issue was, that there were some weird vertices that had bad location data, which showed up as nan (not a number).
Did you import this model from somewhere or created it in Blender?
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Yes I also think something was corrupt with the data. And writing as Obj most likely did remove that,
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I have created it in Blender. No ,there were not weird vertices.
In that video I selected the linked vertices of the head mesh and hidden them. After I selected everything left, it showed some vertices selected and when I focused on them it showed them in the middle of the world with their location in the properties panel showing as nan 
Never encountered this, so no Idea how such vertices can be created.
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Thanks for your response and support
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Hello, I had the same problem now in Blender 3.4.0. I made a save of my .blend file before I tried the solutions, so if anyone is working on fixing this bug then lmk if I should upload my save somewhere. (export import obj fixed it)
And it just happened again, it has to do something about when I switch to work with the reference images, then come back to the model and continue sculpting. (using transparency and only basic brushes grab draw draw sharp and holding shift to smoothen).
If it is a bug and it’s reproduceable, please make a bugerport:
You can upload the (stripped) .blend file directly or if prompted, there. Just be aware that any uploaded .blend file will be publically viewable and be available.
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