Diesel witness flashes and costs to start

Hello everyone, can someone please help me? It's happened to me several times now: the diesel glow plug light comes on and starts flashing, and it's hard to start, emitting white smoke. Once it starts, it continues flashing until about fifteen minutes later, then it stops. I've taken it to the garage and they've fixed it several times, but it only lasted 10 months before happening again. They told me at the garage it's something with the fuse box.

Does anyone know what it could be? Could it be a relay?

Please, I need help.

Thanks.