I investigated this a bit.
It seems that there might be an issue with the new Optimizer class in Gramps 6.0.
Not sure why but this workaround seems to help: add a line in the method apply_logical_op_to_all
in the file gramps/gen/filters/_genericfilter.py
(around line 149) as follows:
optimizer = Optimizer(self)
handles_in, handles_out = optimizer.get_handles()
if not handles_in: handles_in = None # add this line