I get the error that a core property is read-only?
Due to a bug in the HamamatsuHam driver (which is not Open Source, unfortunately, otherwise we could try to fix things), the error messages are not quite stellar. They are even outright misleading.
The issue, most likely, is that Fiji reserves so much RAM for its own use that the Hamamatsu driver is unable to reserve space for its image buffers.
Just go to Edit>Options>Memory & Threads... and limit the RAM Fiji uses to, say, 1GB. Restart Fiji, and all should be fine.