I used to have a Hagen Trio filter, and Hagen also makes Fluvals. The filter I had suffered from a problem - when it was turned off, it wouldn't start again by itself, even if the impeller and the well had been scrubbed sparkling clean. I did solve the problem, but it did leave me worried about power cuts. I had to remove the pump from the filter and start it manually every time I did a water change. It was a bit fiddly at first, but after the first couple of times it only took a few seconds.
I don't know if it would work with your filter but you could try. Looking at the U2 manual, the pump is at the bottom of the filter and the whole thing slots into the bracket. You would have to lift the filter out of the bracket, slip the wire out of its slot and squeeze the release arms on the sides of the motor housing to remove the pump and put the rest of the filter carefully on the tank bottom. Then just turn the electric back on, flick the impeller vanes and if that starts it, push the pump back into the filter, position the wire and slip the filter back into the bracket. Being very careful not to soak yourself in the process - make sure the outflow is pointing downwards at all times till you reseat the filter into the bracket.
If you also want to clean it before restarting, it is much easier to remove the impeller with a pair of tweezers. I have snapped the ceramic shaft in my Eheims by trying to pull it out with my fingers and twisting it. That's why I keep a spare shaft in the cupboard.
Actually, looking at the diagram in the manual more closely, it seems that the impeller sits on a shaft that is itself seated in a plastic holder. Which is exactly how my Trio was. They provided a hook to remove this holder to clean it, but sometimes it was a wrestling match to get it out. And googling the replacement part number, the images look exactly the same as the Trio's. They call it the impeller shaft assembly. Hmmmmm, maybe the U series do suffer from the same problem as the Trio.