I reported this bug some time ago. The phaser effect doesn’t work well if no cabinet is selected.
Create an empty project.
Create an instrument or audio track and put put the VST Rack Amp plugin as an insert effect.
Insert the phaser pedal in and turn it on. Tweak the rate knob, it should work fine.
Now in the cabinet selection choose “No Cabinet”.
Result: The effect then will not work as intended, the LFO rate is extremely fast. If you select a cabinet again it will work.