Same problem,
PS5 uploads fine, PC says cross-save uploading - you can see this in Load Game menu when first launching game. This is aburptly disappears and the syncing message stops after about 10-15 seconds.
It is like the PC connection to Larian servers is dropped or times out.
I relinked accounts, deleted larian account and started again - played with every setting imaginable, in steam, larian account & on router - nothing makes this work.
I would have thought there'd be more uproar about this bug but it seems noone uses cross save after the had to revoke it at the start of october and everyone disabled cross save to actually play the game.