Due to the problem with saves disappearing, I enabled cross-saves as recommended by larian support. My saves were synchronized with the network, this was confirmed when saving and on the save loading screen the corresponding icons were visible for the last five saves.
Today I lost all my saves again and ended up at the initial game startup screen. The problem is that saves from the network are not loaded into the game. I tried turning off the Internet, connecting again, rebooting and changing the account on the xbox and enabling cross-saving again on both the old account and the new one. I also created a new character, and tried to load the save on the creation screen, but it was empty.