Interesting. Sounds like the same symptoms as me - I am also using HW offloading - but hadn't nailed that down to being the cause.
I used the upgrade method provided by @stman
Which seems to have worked fine. However, HW offloading is my primary reason for wanting to use the ERX - so if anyone has any ideas that would be much appreciated!
For now I'm using a pi4 instead - no hardware offloading, but comporable speeds, presumably due to its CPU grunt 