Washing machine connected to a pump constantly loads and unloads watet

Hi, all,
Have interesting one, hope someone can offer some bright ideas…
Moved house. Instalation point for washing machine has been set by previous owners in the basement, with Grundfos Sololigt2 C-3 water pump pushing drainage water up. They swore everything worked properly. Anyway, when we connected our brand new water machine
Hotpoint RDG8643WW 8KG / 6KG 1400 Spin Washer Dryer it was running infinite circle “load water/unload water” for hours if we were not stoping it. It was possible to stop by pausing wash cycle and manually changing it to “spin and end”.
Then we’ve tried to disconnect drainage pipe from pump and fix drainage pipe higher, so it would go up from machine and down to the end of pipe (google said to do so). When we disconnected the pipe from the pump – water leaked from the pump, quite a lot (couple litres for sure). So, we cleaned water, fixed the pipe, connected it with the pump – and all seemed working properly on the test run. We haven’t finished cycle by that time but it was running about 10 minutes without tryng to unload water. But when after few hours we tried to start actual laundry – we had same problem. We disconnected the pipe from the pump (cleared water again), and tried to continue cycle with pipe’s end just left in a bin. It worked properly, we waited around 10 minutes like this – and connected the pipe to the pump. Cycle run smoothly to the end… We tried it couple more times: so when we start cycle with pump – it doesn’t work, it goes to the load/unload circle. But if we will remove pipe and restart – it will continue to work properly. If we will start with the pipe not in the pump – it will work properly and later can be connected to the pump…
Any ideas?
Thank you