The inverter is tripping out.
Under periods of bright sunshine, power flows in the distribution network change or even reverse - such that voltages at your supply point rise. Unless the DNO has adjusted their network to compensate, then the voltage may exceed the upper limit laid down in the standards.
All embedded generation systems are required to have "anti islanding" protection. Basically this means that they must not try and feed an isolated section of network - whether that's just your own house, or the street, or even the whole village - should the bit of the network it's connected to become disconnected from the national grid. In theory, without such protection, it would be possible for a such a section of network to remain powered by the embedded generation with significant risks of equipment damage (eg re-closing a tripped breaker when the voltages aren't synchronised) or injury (eg to DNO personnel working on what should be a dead circuit).
So each device must detect such a situation - and the terminal voltage going outside of limits would be one factor in this. So bright sunshine, voltage rises, inverter trips out until it decides the situation is resolved.
My brother had exactly this problem, and the suppliers adjusted the limits to stop the trips. Go back to the installers, tell them they are spouting male bovine excrement and to adjust the limits. It may mean contacting the DNO and getting them to adjust their network. Either way, your installer should understand this and be able to sort it. I do however suspect that many installers have no idea how any of it works - and so will fob you off with any old story.
