Help required: Earth tripping - but only when connected to grid

Hi all

I am out of ideas - so I am hoping one of you here will be able to help.

The earth on my essential DB trips, but only under certain circumstances. Here the facts as best as I can describe:

• The trips only occur when I am connected to the grid. When I am off-grid, everything runs without problems for weeks
• I have a 63A single-pole smart breaker installed in-between my grid main breaker and my inverter AC In. I use this to disconnect myself from the grid and only re-connect when required (when I see that my PV/batteries will not be sufficient to get me through the day)
• The trips only occur during night-time and early morning. From what I can remember, the earliest trip was around 01h00 and the latest around 07h00
• The trips almost always occur in the 1st night after re-connecting to grid. E.g., last time I re-connected around 23h00 before going to bed and at 01h00 the trip occurred
• The trip is caused by a single circuit. If I flip the earth leakage breaker back on after it has tripped, it immediately trips again (within 30s). If I switch off the circuit breaker for the circuit in question and then switch on the earth leakage breaker again, everything runs stable
• The circuit in question consists of a few plugs inside the house as well as 2 inverter aircons
• The trips occur independent of any circuit (e.g., happens whether any or both aircons are on or off)
• I have physically disconnected everything from the plugs - the trips still occur

I used to have a relay for my earth bond, which my electrician in the meantime has replaced with a permanent bond. The trips occurred with the relay in place (and working) and persist now that I have a permanent bond.

Do any of you have any ideas on what might cause this and how I can fix it?